EXCHANGE 


•li^ls^te 


Examination  of 
Low-Temperature  Coal  Tar 


DISSERTATION 

Submitted  in  Partial  Fulfillment  of  the  Requirements 

for  the  Degree  of  Doctor  of  Philosophy 

in  the  Faculty  of  Pure  Science 

Columbia  University,  in  the 

City  of  New  York 


By 

ROLAND  P.  SOULE,  B.  S.,  Chem.  E.,  A.  M 
New  York  City 

1922 


Acknowledgments 

The  author  takes  pleasure  in  acknowledging  the 
kindly  co-operation  of  Prof.  J.  J.  Morgan,  at  whose 
suggestion  the  present  investigation  was  undertaken, 
and  the  many  courtesies  of  Dr.  H.  A.  Curtis  of  the 
Clinchfield  Carbocoal  Corporation. 


Reprinted  from  Chemical  &  Metallurgical  Ertgineerirife 
Vol.  26,  Nos.  20,  21  and  22,  May  17,  24  and  31,  1922 


Examination  of 

Low-Temperature  Coal  Tar 

I.    Characteristics  of  Low-Temperature  Tar 

Outline  of  a  Scheme  of  Examination  for  Tars  Obtained 
in  the  Low-Temperature  Carbonization  of  Coal — Com- 
position of  a  Commercial  Low -Temperature  Tar  and 
Comparison  With  Coke- Oven  and  Gas- Works  Tars- 
Review  of  Results  Obtained  by  Previous  Investigators 

BY  ROLAND  P.  SOULE 


THE  present  report  is  the  first  of  a  projected  series 
of  studies  in  the  carbonization  of  coal.  It  con- 
cerns itself  primarily  with  an  examination  of  a 
low-temperature  coal  tar,  but  its  scope  has  not  been  con- 
fined to  specific  analytical  data.  In  the  absence  of  any 
standard  analytical  procedure  in  this  little  explored 
field,  it  has  become  necessary  to  develop  a  scheme  of 
examination  by  which  the  component  groups  of  low- 
temperature  tars  may  be  readily  examined.  An  inten- 
sive investigation  has  been  made  of  a  commercial  low- 
temperature  tar,  comparing  the  analytical  results  ob- 
tained with  those  of  other  tars.  This  scheme  and  the 
character  of  the  tar  determined  by  it  are  described  in 
the  present  paper. 

The  composition  of  this  tar  indicates  a  stage  in  the 
decomposition  of  the  primary  liquid  distillates  of  coal 
which  hitherto  has  not  been  carefully  studied.  In  a 
second  installment  it  is  intended  to  define  more  precisely 
several  distillation  reactions,  and  to  present  a  new  view- 
point on  the  much  disputed  mechanism  of  the  carboniza- 
tion of  coal.  Thus,  a  rational  basis  will  be  found  for 
reconciling  to  a  large  extent  the  reported  differences  in 
the  composition  of  low-temperature  coal  tars. 

Until  recently  the  commercial  carbonization  of  coal 
has  been  performed  at  1,000  to  1,300  deg.  C.  primarily 
for  the  production  of  city  gas  and  metallurgical  coke. 
Within  the  past  decade,  however,  lower  temperatures  of 
distillation  have  been  made  to  serve  two  purposes.  The 
first  of  these  is  scientific  in  nature — the  study  of 
the  products  of  distillation  at  various  temperatures  has 
shed  light  on  the  constitution  of  coal,  and  afforded 

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evidence  for  theories  of  carbonization.  The  second  ob- 
ject has  been  the  production  of  new  materials  of  com- 
mercial value.  By  distilling  an  ordinary  bituminous 
coal  at  500  to  600  deg.  C,  there  are  obtained  a  friable, 
free-burning  coke  which  may  be  made  to  serve  as  a 
smokeless  fuel,  and  a  distillate  which  upon  removal  of 
the  phenols  resembles  crude  petroleum  in  properties  and 
uses. 

OUTLINE  OF  INVESTIGATION 

An  outline  of  the  type  of  methods  used  in  the  present 
investigation  is  given  in  Fig.  1.    According  to  this  pro- 


67. 83  7°  Pitch 


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7o  by  Weight  Composition  of  Fraction 

FIG.   2 — COMPOSITION  OF  THE  CARBOCOAL  DISTILLATE 

cedure  the  tar  examined  was  found  to  have  the  com- 
position indicated  in  general  by  Table  I  and  Fig.  2. 

Fig.  2  shows  the  change  in  the  percentages  by  weight 
of  the  various  classes  of  components  in  the  condensate 
as  the  boiling  point  of  the  tar  rises  during  distillation. 
The  vertical  axis  of  boiling  temperatures  is  so  graduated 
that  the  areas  of  the  figure  represent  proportions  by 
weight  of  the  different  groups.  The  horizontal  dotted 
lines  separate  the  five  fractions  of  the  distillate,  and  the 
curved  lines  are  the  boundaries  between  the  component 
groups. 

[3] 


TABLE  I— SUMMARIZED 'COMPOSITION  OF  CARBOCOAL 
DISTILLATE 

Percentages  by  Weight 

Component  Classes  Basis   of   Distillate          Basis  Crude, 

Dry   Tar 

Phenols 42.7  13.7 

Nitrogen  bases 1 . 94  0  624 

Hydrocarbons 55.4  17.8 

Cyclic  unsaturated  (a) 41.5  134 

Saturated 13.9  4.4 

Naphthene 8.8  28 

Paraffine 5.1  1.6 


Totals 100.0     55.4     13.9     32.1          17.8     4.4 

a  May  contain  traces  of  aromatic  hydrocarbons. 


OF  EARLIER  STUDIES 

Low-temperature  coal  tar  may  best  be  defined  as  the 
tar  produced  by  the  carbonization  of  coal  at  tempera- 
tures (500  to  750  deg.  C.)  not  high  enough  to  cause  an 
appreciable  decomposition  of  the  primary  liquid  prod- 
ucts of  distillation. 

Certain  circumstances  must  be  borne  in  mind  in  at- 
tempting a  comparison  of  previous  investigations  of 
low-temperature  tars.  The  low  heat  conductivity  of 
ooal  makes  the  exact  nature  of  tars  distilled  at  approxi- 
mately the  same  temperature  dependent  upon  such 
factors  as  agitation,  rate  of  distillation,  the  thickness 
of  the  fuel  layer  and  the  resulting  temperature  gradient. 
Moreover,  the  extent  to  which  secondary  reactions  may 
also  occur  is  determined  by  the  pressure  employed,  the 
use  of  steam,  the  size  of  the  gas  space,  the  temperature 
of  the  crown  of  the  retort  and  other  conditions  control- 
ling the  path  of  the  gases. 

Of  the  several  investigations  of  one  aspect  or  another 
of  low-temperature  tars,  four  studies  are  cited  in  Table 
II  as  the  most  important  and  representative  efforts 
toward  a  through  analysis.  A.  Pictet1  and  his  co-workers 
in  Switzerland  and  D.  T.  Jones  and  R.  V.  Wheeler2  in 
England  employed  the  tar  as  an  agent  in  their  re- 
searches on  the  constitution  of  coal.  S.  W.  Parr3  and 
his  associates  in  this  country  were  primarily  interested 


'A.  Pictet  et  al.,  Ann.  Chim.  [9],  vol.  10,  p.  249  (1918)  ;  cf.  A. 
Pictet  and  M.  Bouvier,  Ber.,  vol.  46,  p.  3342  (1913)  ;  vol.  48,  p. 
926  (1915)  ;  Compt.  rend.,  vol.  157,  pp.  779,  1436  (1913)  ;  vol.  160, 
p.  629  (1915)  ;  A.  Pictet,  O.  Kaiser  and  A.  Labouchere,  Compt. 
rend.,  vol.  165,  pp.  113,  358  (1917). 

2D.  T.  Jones  and  R.  V.  Wheeler,  J.  Chem.  Soo.,  vol.  105,  p.  140 
(1914). 

3S.  W.  Parr  and  H.  L.  Olin,  Univ.  Illinois  Eng.  Exp.  Sta.  Bulls. 
60  (1912)  and  79  (1915)  ;  T.  E.  Layng,  Dissertation,  Univ.  Illi- 
nois (1915). 

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in  the  commercial  aspects  of  low-temperature  carboni- 
zation, as  were  F.  Fischer4  and  his  staff  in  the  Kaiser- 
Wilhelm  Institute  of  -Coal  Research  in  Mulheim-Ruhr, 
Germany.  The  details  tabulated  in  the  case  of  Fischer's 
work  represent  only  one  of  a  large  number  of  related 
experiments,  but  serve  to  indicate  his  characteristic 
method  of  reporting  tar  investigations  in  technical 
rather  than  in  scientific  terms.  For  the  purpose  of 
comparison  the  results  obtained  in  the  present  investi- 
gation are  also  included  in  this  table. 

ORIGIN  OF  THE  TAR  EXAMINED 

The  tar  examined  was  produced  by  the  "Carbocoal" 
process5  (C.  H.  Smith  patents)  of  the  International 
Coal  Products  Corporation.  It  was  a  representative 
sample  made  in  March,  1921,  under  commercial  con- 
ditions at  the  experimental  plant  of  the  company  at 
Irvington,  N.  J. 

The  low-temperature  tar  produced  by  the  primary 
retorts  is  the  subject  of  this  investigation.  These 
retorts  are  horizontal,  about  18  ft.  long,  and  of  cardioid 
section.  The  walls  are  constructed  of  carborundum 
blocks,  and  the  ends  of  cast  iron  lined  with  firebrick. 
The  coal  is  continuously  stirred  and  advanced  through 
the  retort  by  paddles  of  radius  2  ft.  3  in.  mounted  on 
two  parallel  12-in.  paddle  shafts,  which  revolve  at  1.8 
r.p.m.  Each  retort  is  operated  about  one-third  full,  and 
has  sufficient  capacity  to  permit  an  output  of  1  ton  an 
hour.  The  products  of  distillation  leave  at  the  top  of 
the  discharge  end  of-  the  retort.  The  temperature  in 
the  retort  shell  is  about  730  deg.  C.,  in  the  gas  at  the 
feed  end  of  the  retort  300  deg.  C.  and  at  the  discharge 
end  about  500  deg.  C.  The  temperature  of  the  dis- 
charged coke  approximates  480  deg.  C. 

It  is  apparent  from  this  description  of  retort  con- 
ditions that  the  opportunities  for  secondary  decom- 
position are  not  negligible.  Thus  the  outgoing  products 
of  distillation  come  into  contact  with  surfaces  heated  to 
600  deg.  C.  and  undergo  decomposition  to  some  extent. 

A  Pennsylvania  bituminous  coal  known  as  "Pittsburgh 
Terminal"  coal  was  used  in  the  preparation  of  the 


4F.  Fischer  and  W.  Gluud,  Ges.  Abhandl.  zur  Kenntnis  der 
Kohle,  vol.  1,  p.  114  (1916)  ;  Ber.,  vol.  52,  p.  1035  (1919):  cf. 
also  Ges.  Abhandl,  vol.  3,  pp.  1,  248,  270  (1918). 

6H.  A.  Curtis,  CHEM.  &  MET.  ENG.,  vol.  23,  p.  499  (1920)  ; 
J.  Ind.  Eng.  Chem.,  vol.  13,  p.  23  (1921)  ;  G.  H.  Thurston,  J.  Soc. 
Chem.  Ind.,  vol.  40,  p.  51T  (1921)  ;  Wallace  Savage,  CHEM.  & 
MET.  ENG.,  vol.  19,  p.  579  (1918)  ;  C.  T.  Malcolmson,  Bull.  Am. 
Inst.  Min.  Eng.,  vol.  137,  pp.  971,  1686  (1918). 

[6] 


sample  of  tar  examined.  Its  analysis8  on  a  dry  basis  is 
given  in  Table  II.  The  moisture  content  of  the  coal  as 
fed  into  the  furnace  amounted  to  2  per  cent.  The 
calorific  value  was  13,925  B.t.u.  per  Ib. 

PROPERTIES  OF  THE  TAR  AND  PITCH 

Dehydration  and  Distillation.  Carbocoal  tar,  in  com- 
mon with  other  low-temperature  tars,  is  a  black  oil  of 
petroleum-like  fluidity.  It  smells  strongly  of  cresols 
and  to  a  less  extent  of  hydrogen  sulphide.  The  viscosity 
is  lower  than  that  of  ordinary  coke-oven  and  gas-works 
tar,  and  no  odor  of  ammonia  or  naphthalene  can  be 
detected  in  it. 

The  tar  was  dehydrated  for  subsequent  examination 
by  distilling  to  200  deg.,  separating  the  light  oil  from 
the  water  in  the  condensate  and  returning  it  to  the 
main  body  of  tar  remaining  in  the  still.  A  specially 
constructed  3-gal.  copper  tar-still  of  the  same  propor- 
tions as  the  Barrett  type7  was  used  in  the  operation. 
About  8  kg.  of  tar  was  distilled  at  a  time.  The  content 
of  water  in  the  sample  of  tar  amounted  to  2.12  per  cent. 

A  sample  of  thoroughly  mixed,  crude,  dry  tar  was 
taken  for  specific  gravity  (Hubbard  bottle)  and  "free 
carbon"  determinations  according  to  standard  proce- 
dures7. A  dry  distillation  of  the  8  kg.  of  crude  tar  thus 
dehydrated  was  made  at  atmospheric  pressure  in  the 
same  copper  still.  The  rate  was  15  c.c.  per  minute,  or 
about  0.2  per  cent  of  the  total  volume  of  tar  per  minute. 
For  comparison  samples  of  coke-oven  tar8  and  gas- 
works tar9  were  fractionated  in  exactly  the  same  man- 
ner and  the  results  are  recorded  in  Fig.  3  and  Table  III. 

In  general  there  is  a  close  resemblance  between  the 
three  fractionation  curves,  disproving  the  many  exag- 
gerated statements  that  have  been  made  of  the  larger 
quantity  of  low-volatile  oils  in  low-temperature  coal 
tars.  The  percentage  of  pitch  is  only  slightly  lower 
in  this  tar  than  in  the  high-temperature  tars,  although 
the  difference  would  be  greater  with  higher  cutting 
temperatures,  or  if  the  distillates  had  been  carried  to 


6Analysis  by  Research  Laboratory  of  the  International  Coal 
Products  Corporation. 

7J.  M.  Weiss,  J.  Ind.  Eng.  Chem.,  vol.  10,  pp.  732,  817   (1918). 

"Obtained  through  the  courtesy  of  the  Seaboard  By-Products 
Coke  Co.,  Kearny,  N.  J.  (Koppers  retorts)  ;  distilled  at  850-900 
deg.  from  a  mixed  coal,  avg,  30  per  cent  V.C.M. 

•Obtained  through  the  courtesy  of  the  Consolidated  Gas  Co., 
New  York  City ;  consists  of  approximately  equal  proportions  of 
horizontal  and  inclined  retort  tars  ;  coal  distilled  at  about  1,000 
deg. 

[7] 


TABLE  III— FRACTIONATIONS  OF  REPRESENTATIVE  TARS™ 


B.P.  Range             Low-Temperature         Coke-Oven             Gas^Worke 

Deg.  C. 

Tar 

Tar 

Tar 

20-173 

0.64 

0.70 

1.19 

173-237 

9.19 

8.27 

9.22 

237-281 

12.50 

12.44 

10.12 

281-315 

6.94 

6.21 

3.78 

315-326 

2.90 

2.53 

1.93 

Pitch    (diff.) 

67.83 

69.85 

73.76 

M. 

P.  of  pitch,  Deg.  C. 

53 

69 

89 

a  pitch  of  the  same  melting  point  in  each  case,  as 
commercial  conditions  might  demand.  Thus,  when  Car- 
bocoal  tar  is  carried  to  a  pitch  melting  at  80  deg.  C. 
for  use  in  briquetting,  only  56  per  cent  remains  in  the 
still. 

Pitch.  The  pitch  obtained  from  the  low-temperature 
tar  was  an  amorphous,  black  residue,  which  lacked  the 
brittleness  and  lustrous  fracture  of  high-temperature 
pitches.  Briquets  made  from  it  lost  their  sharp  edges 


S  10  IS  20  25  30  35 

Percent  by  Vo/ume  Crude  Tar  Distilled 

FIG.    3— DISTILLATION   OF   CRUDE.   DRY   TARS 


TABLE  IV—  COMPARISON  OF  TARS  AND  PITCHES 

Low- 

Coke- 

Gas- 

Temperature 

Oven 

Works 

ude,  dry  tar 

Sp  gr  155  deg  /  1  5  5  deg 

1   0676 

1.1845 

1.2172 

"Free  carbon,"  per  cent                 .  . 

....             0.71 

6.93 

20.1 

t'itch,  cut  at  326  deg. 

Sp.gr.  I5.5deg./I5.5deg  
"Free  carbon  "  per  cent 

1.134 

1.263 
16.8 

1.312 

31.  \ 

2.17 

53 

69 

89 

10A11   temperatures   reported   in   this   investigation   are   corrected 
for  the  emergent  stems  of  the  thermometers. 

[8] 


within  an  hour  at  room  temperature.  The  specific  grav- 
ity (Hubbard  bottle),  "free  carbon"  and  air  melting 
points  of  the  Carbocoal,  coke-oven  and  gas-works  tars 
and  pitches  were  determined  by  standard  procedures, 
and  are  recorded  in  Table  IV. 

The  Phenols 

The  high  content  of  phenols  in  low-temperature  tars 
has  been  remarked  by  nearly  all  investigators  (c/. 
Table  II),  and  within  certain  limits  may  be  accepted 
as  an  important  characteristic  distinguishing  them 
from  high-temperature  tars. 

Removal  From  the  Tar.  The  percentages  of  phenols 
were  determined  separately  in  each  of  the  five  frac- 
tions (cf.  Table  III).  In  the  method  used,  100  g.  of  a 
fraction  was  weighed  into  a  200-c.c.  glass-stoppered 
separatory  funnel,  and  extracted  four  times  each  with 
50  c.c.  of  10  per  cent  sodium  hydroxide  solution.  The 
loss  in  weight  of  each  sample  checked  against  the  gain 
in  weight  of  the  alkali  used  was  taken  as  the  percentage 
of  tar-acids  present. 

Fig.  4  was  obtained  by  plotting  percentages  of  tar- 
acids  against  the  average  boiling  points  during  distil- 
lation of  the  corresponding  tar  fractions,  taken  as  the 
mean  ordinates  of  the  respective  sections  of  the  distilla- 


/40 


180 


220 


260 


300 

Average  B.  P.  of  Fractions      °C. 

FIG.  4 — RELATION  OF  THE  BOILING  POINTS  OF 

TAR  FRACTIONS  TO  PHENOL 

CONTENT  OF  EACH 

[9] 


tion  curve  of  Fig.  3.  It  shows  a  well-defined  maximum 
between  240  and  280  deg.,  and  indicates  that  a  large 
quantity  of  tar  acids  is  contained  in  the  pitch. 

Purification  of  the  Tar- Acids.  The  fractions  of  low- 
temperature  tar  were  washed  completely  free  of  phenols 
with  10  per  cent  sodium  hydroxide  solution.  The  alka- 
line solution,  after  purification  by  benzene  extraction 
and  steam  distillation,  was  acidified  with  20  per  cent 
sulphuric,  acid  to  liberate  the  phenols,  as  indicated  by 
Fig.  1.  The  tar-acids  thus  recovered  formed  a  dark 
brown,  slightly  viscous  liquid,  which  had  a  strong  odor 
of  cresols. 

Carboxylic  Acids.  No  solubility  of  this  mixture  in 
a  saturated  sodium  carbonate  solution  could  be  de- 
tected, and  a  sample  after  solution  in  sodium  hydroxide 
was  completely  recovered  by  precipitation  with  carbon 
dioxide.  Carboxylic  acids,  therefore,  could  be  admixed 
with  the  phenols  only  to  a  negligible  extent. 

None  of  the  four  investigators  quoted  above  has  re- 
ported such  compounds,  but  recently  Marcusson  and 
Picard11  found  in  a  low-temperature  tar  from  upper 
Silesian  coal  a  solid  mixture  of  13  per  cent  phenols  and 
12  per  cent  aromatic  carboxylic  acids.  Tropsch13  dis- 
tilled under  similar  conditions  a  coal  from  the  same 
region,  however,  and  found  viscous,  not  solid  phenols, 
with  only  traces  of  solid  carboxylic  acids.  He  stated 
that  the  tar  described  by  Marcusson  and  Picard  was 
probably  exceptional. 

Phenol  Ethers. — The  occurrence  of  guaiacol  in  wood- 
tar"  and  in  peat-tar,14  both  of  which  are  formed  by 
carbonization  below  500  deg.  and  resemble  low-tem- 
perature coal  tars  in  their  high  content  of  phenols,  sug- 
gests the  possible  presence  in  the  latter  of  phenol 
ethers.  A  negative  Zeisel  test,  however,  showed  that 
methoxyl  and  ethoxyl  groups  are  absent  from  the 
phenol  mixture  obtained  from  the  low-temperature  tar. 

A  similar  result  has  been  reported15  for  the  phenols 
from  Lohberg  coal  tar  (cf.  Table  II). 

UJ.  Marcusson  and  M.  Picard.  Z.  angew.  Chem.,  vol.*  341,  p.  201 
(1921). 

"Hans  Tropsch,  Brennstoff  Chem.,  vol.  2,  p.  251  (1921)  ;  cf. 
ibid.,  vol.  2,  p.  312  (1921),  where  he  reports  0.49  per  cent  of  car- 
boxylic acids  in  a  vacuum  tar  from  Lohberg  coal. 

"Ossian  Aschan,  Brennstoff  Chem..  vol.  2,  p.  273  (1921)  reports 
4.9  per  cent  of  phenol  ethers  in  wood  tar. 

ME.  Bornstein  and  P.  Bernstein,  Z.  angew.  Chem.,  vol.  271,  p. 
71  (1914)  ;  F.  M.  Perkin,  /.  Soc.  Chem.  Ind.,  vol.  33,  p.  395 
(1914)  ;  J.  Inst.  Pet.  Tech.,  vol.  1,  p.  76  (1914). 

«W.  Gluud  and  P.  K.  Breuer,  Oes.  Abhandl.,  vol.  2,  p.  236 
(1917). 

[10] 


Polyhydroxy  Phenols.  The  water  obtained  simulta- 
neously with  the  tar  in  the  distillation  of  the  coal  in 
the  low-temperature  process  was  not  available  for  the 
present  investigation.  Since  this  liquor  contained  the 
largest  proportion  of  polyhydroxy  phenols  distilled 
from!  the  coal,  their  estimation  in  the  tar  was  not 
attempted. 

Catechol  has  been  identified16  among  the  products  of 
low-temperature  distillation  of  Lohberg  coal,  and  was 
estimated  to  be  present  to  the  extent  of  0.02  per  cent 
of  the  weight  of  the  coal. 

FRACTIONATION  AND  DENSITY  DETERMINATIONS 

The  phenol  mixture  from  the  low-temperature  tar 
had  a  specific  gravity"  of  1.036  at  25  deg./4  deg.  as 
compared  with  1.044  of  the  mixed  coke-oven  and  gas- 
works phenols  extracted  with  10  per  cent  sodium  hy- 
droxide at  60  deg.,  and  purified  in  the  same  manner  as 
the  low-temperature  phenols.  Fractionation  curves  of 
the  two  types  are  seen  in  Fig.  5. 

It  is  evident  that  in  the  first-named  mixture  there 
are  no  phenols  of  boiling  point  higher  than  may  be 
found  among  the  high-temperature  phenols.  The  pro- 
portions of  the  components,  however,  differ  quite 
markedly,  52  per  cent  by  volume  of  the  low-temperature 
phenols  boiling  below  220  deg.,  as  opposed  to  77  per 
cent  of  the  phenols  from  coke-oven  and  gas-works  tar. 
In  both  cases  these  percentages  may  be  increased  con- 
siderably by  continued  fractionation.  • 

To  obtain  a  more  informative  comparison  of  the  low- 
and  high-temperature  phenols,  each  series  was  slowly 
distilled  four  times  with  a  Young  four-pear  head,  divid- 
ing the  distillate  into  eight  fractions.  The  specific 
gravity  at  25  deg./4  deg.  of  each  was  determined,  and 
plotted  in  Fig.  6  against  the  average  boiling  point  of 
that  fraction,  taken  as  the  mean  ordinate  of  the  boiling- 
point  curve  of  the  final  fractionation  in  each  case. 

The  decrease  in  the  specific  gravity  of  the  phenols  as 
the  boiling  point  increases  indicates  the  presence  of 
aliphatic  sidechains  of  lower  density  attached  to  the 
phenol  nucleus — e.g.,  cresols  and  xylenols.  In  the 
higher  homologs  a  sharp  rise  in  density  and  a  notable 


18W.  Gluud,  Ges.  Abhandl.,  vol.  3,  p.  66  (1918)  ;  cf.  W.  Gluud 
and  P.  K.  Breuer,  loc  cit.  (ref.  15),  and  E.  Bornstein,  Ber.,  vol. 
35,  p.  4324  (1902). 

"Unless  otherwise  specified,  all  densities  cited  in  this  investiga- 
tion were  determined  by  use  of  a  Becker  analytical  balance  and 
a  7  g.  glass  plummet  immersed  in  the  liquid,  which  was  jacketed 
with  water  at  the  desired  temperature. 


TABLE  V—  FRACTIONATION 

OF  PHENOLS  FROM  LOW- 

TEMPERATURE 

TAR 

Fraction, 

Weight 
in  G., 

Per  Cent 
of  Total 

Fraction, 

Weight 
in  G., 

Per  Cent 
of  Total 

Deg.  C. 
182-189 

Fraction 
41 

Phenols 
7.3 

No. 
6 

Deg.  C. 
214-220 

Fraction 
44 

Phenols 
7.9 

189-195 

38 

6.8 

7 

220-227 

14 

2.5 

195-202 

60 

10.8 

8 

227-260 

102 

18.3 

202-207 

71 

12.7 

9 

260-300 

92 

16.5 

207-214 

48 

8.6 

10 

Pitch 

48 

8.6 

Total 


558 


100.0 


increase  in  the  viscosity  of  the  fractions  mark  the 
appearance  of  a-  and  /3-naphthols18  in  the  high-tem- 
perature phenols,  and  bicyclic  compounds  at  least  in  the 
low-temperature  phenols. 


/O    20    30    40    50    60    70     80    90    100 

Percentage  of  Total  Vo/ume  Distilled 

FIG.    5— DISTILLATION    OF    PHENOLS 

These  higher  homologs  have  been  investigated,1" 
and  found  to  contain  naphthol  derivatives,  as  the  wide 
divergence  of  the  two  curves  suggests,  but  neither  a- 
nor  ^-naphthol.  However,  the  coincidence  of  the 


18K.  E.  Schulze,  Lieb.  Ann.,  vol.  227,  p.   143    (1885). 

"Unpublished  investigation  of  M.  T.  Bogert  and  S.  Caplan.  to 
whom  are  due  the  data  in  Fig.  6  on  the  three  highest-boiling 
fractions. 


[12] 


first  section  of  the  curves  indicates  that  the  Carbocoal 
mixture  contains  the  same  low-boiling  components  as 
the  high-temperature  phenols,  although  the  proportion 
is  much  smaller. 

Another  sample  of  the  mixed  phenols  was  fraction- 
ated once  each  with  a  Young  four-pear  and  a  10-in. 
Vigreux  column,  and  then  five  additional  times  below 
230  deg.  with  the  Vigreux  head.  The  average  rate  of 
distillation  was  1  c.c.  per  minute,  or  0.2  per  cent  by 
volume  per  minute.  The  results,  corrected  for  the 
slight  distillation  losses,  are  recorded  in  Table  V. 

The  specific  gravity  and  bromine  tests  of  Fraction  1 
indicated  the  presence  of  phenol.  Fraction  2  showed 
with  ferric  chloride  the  characteristic  color  reaction  of 
o-cresol.  In  classifying  these  fractions  it  is  customary 
to  assume  that  if  the  cut  is  made  midway  between  the 
boiling  points  of  two  components,  the  quantity  of  the 
higher-boiling  compound  appearing  in  the  lower  frac- 
tion will  be  approximately  balanced  by  an  equal  amount 
of  the  lower-boiling  compound  in  the  higher  fraction. 
Subsequent  experiments  have  shown  this  assumption 
to  be  justified.  On  this  basis  Fractions  1  through  4 
contain  phenol  and  the  cresols  (210  g. ;  37.6  per  cent); 
Fractions  5  through  7  contain  the  xylenols  (106  g.; 
19.0  per  cent)  ;  and  Fractions  8  through  10  contain  the 
higher  homologs  (194  g. ;  34.8  per  cent). 

ESTIMATION  OF  PHENOL  AND  THE  CRESOLS 

The  general  scheme  employed  is  that  indicated  in 
Fig.  1.  The  phenol  and,  cresols,  separated  from  the 
higher  homologs  by  15  fractionations,  amounted  to 
35.0  per  cent  (uncorrected  for  distillation  losses;  cf. 
37.6  per  cent  above)  of  the  total  phenols.  Since  m-  and 
p-cresols  preponderated  in  the  mixture  thus  obtained, 
pure  o-cresol  was  added  to  accomplish  a  sharper  separa- 


TABLE  VI— COMPOSITION  OF  THE  LOW-TEMPERATURE  PHENOLS 

Percentages  by  Weight, 
Basis  of 

Distil-  Crude 

Component                                                                         Phenols         late  Tar 

Phenol 4.2           1.9  0.6 

Cresols  a                                              33.4          15.2  4.9 

Xylenol  fraction 19.0           8.7  2.8 

Higher  homologs 34.8         15.9  5.1 

Pitch  (acid resins) 8.6           3.9  1.3 

Totals 100.0         45.6         14.7 

a  Ratio:  27  per  cent  ortho-,  19  per  cent  meta-,  and  54  per  cent  para-cresols. 


[13] 


tion  by  distillation  into  two  fractions,  one  comprising 
essentially  phenol  and  o-cresol,  and  the  other  a  mixture 
of  the  three  cresols. 

These  fractions  were  analyzed  separately  by  methods 


1.120 


1.000 

160    200    240    280    320 

^rerage  Boiling  Point,    °C. 

FIG.   6— BOILING  POINT  VS.  SPECIFIC  GRAVITY 
OF  PHENOLS 

involving  nitration20  and  the  determination  of  density 
and  freezing  points.21  Table  VI  shows  the  composition 
of  these  phenols. 

COMPARISON  WITH  PREVIOUS  INVESTIGATIONS 

The  yield  of  tar  in  the  Carbocoal  process  from  the 
particular  coal  in  question  is  225  Ib.  per  ton,  or  11.3 
per  cent.  This  is  higher  than  any  other  yield  reported 
in  Table  II.  The  reason  for  this  lies  to  a  large  extent 
in  differences  in  the  composition  of  the  coal  used  by 
various  investigators,  as  indicated  by  Table  VII. 


XF.  Raschig.  Z.  angew.  Chcm.,  vol.  14,  p.  759   (1900). 

^J.  J.  Fox  and  M.  F.  Barker,  J.  Soc.  Chem.  Ind.,  vol.  36,  p.  842 
(1917)  ;  vol.  37,  265T,  268T  (1918)  ;  vol.  39,  p.  169T  (1920)  ; 
H.  M.  Dawson  and  C.  A.  Mountford,  J.  Chem.  Soc.,  vol.  113,  pp. 
923,  935  (1918).  J.  M.  Weiss  and  C.  R.  Downs,  J.  Ind.  Eng. 
Chem.,  vol.  9,  p.  569  (1917)  ;  G.  W.  Knight,  C.  T.  Lincoln,  G. 
Formanek  and  H.  L.  Follett,  ibid.,  vol.  10,  p.  9  (1918). 

[14] 


TABLE  VII— COMPARATIVE  YIELDS  OBTAINED  IN  VARIOUS 
INVESTIGATIONS 


Investigation 
Pictet   

Temp, 
of 
Distil- 
lation, 
Deg.   C. 
450  max. 
430 

Time 
of 
Distil- 
lation 
5hr. 
5  wks. 
6-8  hr. 
3hr. 
1-2  hr. 

Tar 
PerCent    Yield 
V.C.M.  PerCent       PerCent 
in         Wgt.,           Tar-Acid 
Coal       Coal        Dist.       Tar 
15-20        4.0                          8 
26-31         6.5        12-15      6-8 
35           8.7          40          28 
35         11.3           43           13 
39         10.  0           ..           50 

Jones  &  Wheeler 

Parr 

450-525 

Morgan  &  Soule  
Fischer  

...     500-600 
.  .  .    350-500 

For  the  purpose  of  comparison  it  may  be  noted  in 
connection  with  this  table  that  the  distillation  of  a 
30-35  per  cent  volatile  coal  in  a  coke  oven  at  900-1,000 
deg.  C.  yields  from  4  to  6  per  cent  of  tar.  The  phenols 
recovered  from  the  coke-oven  and  gas-works  tar  men- 
tioned above  averaged  about  9  per  cent  by  weight  of 
the  distillate,  or  2.5  per  cent  of  the  total  tar. 

It  must  be  borne  in  mind  in  studying  these  results 
that  Pictet  and  Fischer  alone  actually  removed  the 
phenols  from  the  entire  tar,  and  that  the  percentages  of 
tar-acids  in  the  total  tars  reported  by  the  others  are 
consequently  somewhat  too  low.  As  an  added  complica- 
tion, Parr3  reports  a  decrease  in  the  yield  of  tar-acids 
with  a  decrease  in  the  amount  of  steam  used  in  the 
distillation  of  the  coal.  In  general,  however,  it  will  be 
observed  that  the  content  of  phenols  increases  with  a 
rise  in  the  volatile  content  of  the  coal  from  which  it 
was  distilled,  and  hence  with  the  actual  yield  of  tar. 
It  seems  apparent,  therefore,  that  this  increase  in  bulk 
of  the  tar  is  due  largely  to  phenols. 

The  comparison  of  Table  VII  is  superficial,  however, 
and  the  underlying  cause  of  this  relationship  is  prob- 
ably not  so  simple  a  matter  as  the  mere  volatile  con- 
tent. It  has  been  shown22  that  the  phenols  owe  their 
origin  to  the  fraction  of  coal  insoluble  in  pyridine  and 
chloroform  (the  so-called  "cellulosic"  constituent), 
which  is  not  a  direct  function  of  the  volatile  content. 
A  low  content  of  phenols  in  a  tar,  therefore,  cannot 
be  taken  as  direct  evidence  of  a  high  temperature  of 
coal  carbonization  unless  the  constitution  of  the  coal 
distilled  is  also  taken  into  consideration. 

PREVIOUS  ANALYSES  OF  LOW-TEMPERATURE  PHENOLS 

Jones  and  Wheeler2  collected  the  fraction  of  phenols 
distilling  between  100  and  145  deg.  C.  at  30  mm.  (d. 

^D.  T.  Jones  and  R.  V.  Wheeler,  J.  Chem.  Soc.,  vol.  107,  p.  1318 
(1915)  ;  vol.  109,  p.  707  (1916)  ;  cf.  S.  R.  Illingworth,  J.  Soc. 
Chem.  Ind.,  vol.  39,  p,  HIT  (1920). 

[15] 


1.037  at  15  deg./15  deg.),  and  from  its  ultimate 
analysis  concluded  it  to  consist  essentially  of  a  mixture 
of  cresols  and  xylenols.  Pictet1  stated  that  the  phenols 
obtained  in  fresh  Montrambert  "vacuum  tar"  have  a 
very  high  boiling  point,  crystallize  easily  and  contain 
no  phenol,  cresols  or  xylenols.  Among  the  phenols 
which  appeared  on  standing  in  a  sample  of  tar  prepared 
5  years  before,  he  was  able  to  identify  phenol  itself, 
the  three  cresols  and  1:2:4  xylenol  by  qualitative 
tests. 

Fischer*3  reported  the  45  per  cent  of  phenols  from 
Lohberg  gas  coal  to  contain  0.25  per  cent  catechol,  0.06 
per  cent  phenol,  1  to  2  per  cent  cresols,  1  to  2  per  cent 
xylenols,  30  to  32  per  cent  higher-boiling  than  xylenols, 
and  10  per  cent  acid  resins.  Phenol  was  determined  by  a 
freezing-point  method"  in  a  mixture  obtained  by  frac- 
tional neutralization  of  the  tar-acids.  The  cresols  were 
studied  by  a  development  of  Lederer's  chloracetic-acid 
method25  using  Raschig's  nitration  method20  as  an 
alternative  procedure  for  m-cresol.  The  amount  of 
m-cresol  was  about  one-quarter  of  the  total  cresols, 
which  checks  roughly  the  19  per  cent  found  in  the 
cresols  from  low-temperature  tar. 

The  Nitrogen  Bases 

Percentage  of  Bases.  The  percentages  of  bases  in 
low-temperature  tar  were  determined  in  four  of  the  five 
phenol-free  fractions  of  the  tar  distillate  by  a  gravi- 
metric method  similar  to  that  used  for  the  tar-acids 
(cf.  Fig.  1).  Since  the  quantity  of  bases  contained 
in  the  first  fraction  (20-173  deg.)  was  too  small  for 
quantitative  extraction  with  20  per  cent  sulphuric  acid, 
however,  recourse  was  taken  to  a  titration  method, 
using  methyl  orange  as  indicator.26 

The  results  of  these  analyses  are  recorded  in  Fig. 
7,  where  the  percentages  of  bases  in  each  tar-fraction 
are  plotted  against  the  average  boiling  point  of  that 
fraction  during  distillation.  The  rapid  rise  of  this 
curve  indicates  that  the  pitch  probably  contains  a 
greater  quantity  of  these  compounds  than  the  distillate. 

»F.  Fischer,  Brennstoff  Chcm.,  vol.  1,  pp.  31,  47   (1920). 

"Franz  Fischer  and  P.  K.  Breuer,  Oes.  Abhandl.,  vol.  3,  p. 
82  (1916)  ;  cf.  ibid..,  vol.  2,  p.  236  (1917)  ;  F.  Fischer  and  H. 
Groppel,  Z.  angew.  Chem..  vol.  301.  p.  76  (1917)  ;  Oes.  Abhandl, 
vol.  2,  p.  178  (1917). 

MLu  Lederer,  Frdl,  vol.  4,  p.  91    (1894/1897)  ;  D.R.P.  79,514. 

^W.  Gluud  and  P.  K.  Breuer,  Gea.  Abhandl.,  vol.  3,  p.  227 
(1918). 

[16] 


Examination  and  Classifications.  The  crude  tar-bases, 
recovered  and  purified  as  indicated  in  Fig.  1,  formed  a 
non-viscous,  dark  brown  liquid  mixture  with  a  strong 
odor  of  pyridine.  Having  ascertained  that  the  decom- 
position resulting  was  negligible,  a  sample  was  rapidly 
distilled  to  dryness  to  obtain  a  clear  distillate  for  the 
later  tests. 

The  bases  showed  a  negative  carbylamine  reaction, 
and  with  nitrous  acid  evolved  insufficient  nitrogen  to 
be  measured.  Primary  bases  could  therefore  be  present 
only  to  a  negligible  extent.  A  saturated  solution  of 
sodium  nitrite  precipitated  a  mixture  of  nitrosamines 
from  a  hydrochloric  acid  solution  of  the  bases.  After 
purification  their  nature  was  verified  by  the  Lieber- 
mann  and  diphenylamine  reactions.  The  tests  were 
both  pronounced,  and  demonstrated  the  presence  of 
secondary  amines. 

Hinsberg's  benzene-sulphonyl-chloride  method  was 
used  to  separate  the  secondary  from  the  tertiary 
bases.  A  precipitate  of  sulphonamides  was  formed, 


140         180          220         260         300          340 

Average  B.  P.  of  Fractions,  °C. 

FIG.  7— RELATION  OF  BOILING  POINT  OF  TAR 

FRACTIONS  TO  PERCENTAGE  OF  BASES 

CONTAINED  IN  EACH 

[17] 


which  was  completely  insoluble  in  alkali.  On  acidifica- 
tion, 80  per  cent  of  the  sample  passed  into  the  acid 
layer.  The  Carbocoal  bases,  therefore,  contained  no 
primary,  about  20  per  cent  secondary,  and  80  per  cent 
of  tertiary  bases.  The  purified  tertiary  bases  recov- 
ered by  this  method  and  the  sulphonamides  prepared 
from  the  secondary  bases  rapidly  reduced  a  1  per  cent 
aqueous  solution  of  potassium  permanganate,  indicat- 
ing unsaturation. 

Fractionation  and  Specific-Gravity  Determinations. 
The  density  of  the  mixture  of  low-temperature  bases 
was  0.993,  and  that  of  the  high-temperature  bases  was 
1.060  at  15.5  deg./4  deg.  C.  Comparative  fractional 
distillations  showed  that  the  bases  from  low-tempera- 
ture tar  are  lacking  in  any  preponderant  single  com- 
ponent analogous  to  quinoline  in  the  high-temperature 


'/40          180          220         260          300        340 

Average  B.  R  of  Fractions,  °C. 

FIG.    8— BOILING  VS.    SPECIFIC    GRAVITY 
OF  BASES 


bases.  Following  the  practice  employed  with  the  tar- 
acids,  the  bases  were  re-fractionated  with  a  10-in. 
Vigreux  column,  and  six  fractions  obtained  in  each 
case.  The  specific  gravities  of  the  fractions,  deter- 
mined after  drying  over  solid  potassium  hydroxide,  are 

[18] 


plotted  in  Fig.  8  against  the  corresponding  average 
boiling  points  (determined  graphically).  These  curves 
demonstrate  that  the  low-temperature  bases  are  in 
general  quite  different  in  composition  from  those  in 
ordinary  coke-oven  and  gas-works  tar.  This  is  con- 
trary to  the  case  of  the  tar-acids  (Fig.  6). 

The  smallest  difference  in  density  between  the  low- 
and  high-temperature  bases  occurs  in  the  low-boiling 
fractions.  The  first  fraction  of  the  low-temperature 
bases  had  a  strong  odor  of  pyridine,  and  was  the  only 
fraction  appreciably  soluble  in  water.  With  a-dinitro- 
chlorbenzene  it  showed  the  characteristic  red-violet 
color  of  the  pyridine  derivative.  In  common  with  ordi- 
nary tar,  therefore,  low-temperature  tar  contains  some 
pyridine. 

In  the  distillates  between  230  and  260  deg.  the 'densi- 
ties of  the  bases  are  about  0.07  lower  than  those  of 
the  corresponding  fractions  of  the  quinoline-containing 
high-temperature  bases.  This  difference  indicates  in 
the  low-temperature  bases  a  greater  degree  of  hydro- 
genation  of  the  nucleus,  the  presence  of  aliphatic 
side-chains  of  higher  molecular  weight,  or  both.  Such 
compounds  are  exemplified  by  point  a,  N-methyl- 
tetrahydroquinoline  and  by  point  b,  2  methyl,  3  ethyl, 
1:4:5:6  tetrahydropyridine.  Since  unsaturated  bases 
are  also  present,  compounds  of  the  type  of  dihydro- 
quinoline  may  occur  in  the  mixture. 

Average  Molecular  Weights.  The  relative"  molec- 
ular weights  of  the  various  fractions  of  low-  and 
high-temperature  bases  were  determined  by  the  ordi- 
nary Beckmann  cryoscopic  method,  using  benzene  as 
the  solvent.  It  was  found  that  as  the  boiling  points  of 
the  fractions  increase  the  difference  between  the  re- 
spective molecular  weights  varies  in  much  the  same 
fashion  as  the  difference  in  specific  gravity  (Fig.  8), 
the  maximum  variation  being  15  to  20  in  the  middle- 
boiling  fractions.  Hence  the  lower  densities  of  the 
Carbocoal  bases  seem  chiefly  ascribable  to  the  presence 
of  aliphatic  side-chains  of  molecular  weight  higher 
than  those  of  the  high-temperature  bases.  This  con- 
clusion gains  weight  from  a  consideration  of  the 
analogous  relationships  between  the  principal  hydro- 
carbons of  low-  and  high-temperature  tars  discussed 
below. 

Previous  Investigations.     Jones   and   Wheeler2  were 


"Pyridine  bases  are  slightly  associated  in  benzene  solution. 

[19] 


unable  to  obtain  more  than  traces  of  nitrogen  bases  in 
the  small  amount  of  tar  at  their  disposal.  Parr3  re- 
ported an  average  of  0.23  per  cent  of  "amines"  on  the 
basis  of  the  crude,  dry  tar  in  the  fractions  boiling 
below  260  deg.  C.  Gluud28  announced  0.46  per  cent  of 
"pyridine"  in  the  Lohberg  coal  tar  mentioned  above 
(Table  II).  These  investigators  did  not  interest  them- 
selves in  the  nature  of  the  bases. 

Pictet1  found  0.2  per  cent  of  bases  in  the  "vacuum 
tar"  of  Montrambert  coal.  He  reported  primary  amines 
to  be  present,  and  identified  the  fraction  198-203  deg. 
as  a  mixture  of  toluidines.  Pyridine  and  other  tertiary 
amines  appeared  to  be  absent.  Ultimate  analysis  of 
the  picrates  of  the  higher  fractions  showed  them  to  be 
the  dihydro-derivatives  of  quinoline  and  iso-quinoline. 
The  significance  of  the  absence  of  tertiary  bases  in  this 
tar  and  their  presence  in  low-temperature  tar  is  dis- 
cussed in  the  section  on  secondary  carbonization 
reactions. 

Alcohol  and  Sulphur  Compounds 

Alcohols.  A  sample  of  the  low-temperature  distillate 
freed  of  tar-acids  and  bases  was  boiled  with  metallic 
sodium,  but  no  evolution  of  hydrogen  or  loss  in  weight 
of  sodium  was  observed.  Alcohols  are  therefore  absent. 
Pictet2  found  2  per  cent  of  alcohols  in  "vacuum  tar." 
The  lowest-boiling  component  proved  to  be  hexahydro- 
p-cresol;  the  others  were  isomers  of  monatomic 
phenols,  a  mixture  of  which  they  yielded  on  simple 
standing  for  a  month.  No  other  investigator  has  re- 
ported alcohols  in  low-temperature  coal  tars. 

Sulphur  Compounds.  Hydrogen  sulphide  was  deter- 
mined (0.08-0.1  per  cent)  in  the  first  two  fractions  of 
the  low-temperature  distillate,  but  most  of  this  com- 
pound is  to  be  found  in  the  gaseous  and  aqueous  prod- 
ucts of  distillation.  A  negative  phenylhydrazine  test 
showed  the  absence  of  carbon  disulphide.  Thiophene 
is  indeterminate  by  the  indophenine,  thallin  and  mer- 
curic salt  tests  in  the  presence  of  the  large  proportion 
of  unsaturated  hydrocarbons,  which  cannot  be  removed 
alone.  No  analyses  of  sulphur  compounds  are  reported 
in  the  literature. 

The  Saturated  Hydrocarbons 

Examination  of  the  Hydrocarbon  Mixture.  The 
neutral  oils  remaining  after  the  extraction  of  the  tar- 
acids  and  bases  constitute  the  hydrocarbons  of  low- 

L20J 


temperature  tar.  When  freshly  distilled,  the  lower 
fractions  are  pale  yellow  and  the  higher  fractions 
amber  colored.  After  standing  for  several  weeks,  all 
of  the  fractions  become  quite  black  in  color,  and  de- 
posit on  the  walls  and  bottoms  of  the  bottles  a  thin, 
black  coating.  This  is  evidently  due  to  the  spontaneous 
polymerization  or  oxidation  of  a  very  small  portion 
of  the  unsaturated  compounds  present,  and  is  more 
marked  in  the  lower  than  in  the  higher  fractions.  The 
density  of  fraction  20-173  deg.  increased  0.012  on 
standing  for  4  months. 

The  specific  gravity  of  the  total  mixed  hydrocarbons 
in  the  fresh  distillate  was  found  to  be  0.891  at  25 
deg./4  deg.  as  opposed  to  1.028  of  the  coke-oven  hydro- 
carbons distilled  under  similar  conditions.  Fractional 
distillation  of  the  Carbocoal  hydrocarbons  shows  the 
presence  of  no  preponderant  single  compound  such  as 
the  naphthalene  of  high-temperature  tar.  Since  only 
a  few  very  fine  flakes  of  white  crystals  could  be  ob- 
tained in  fractions  cooled  to  — 30  deg.  C.,  only  traces 
of  solid  aromatic  hydrocarbons,  such  as  naphthalene 
or  anthracene,  can  be  present  in  the  low-temperature 
tar. 

Composition  of  the  Mixture:  Separation  of  the 
Classes  of  Compounds.  All  fractions  of  the  hydrocar- 
bons from  low-temperature  tar  instantly  reduced  a  1 
per  cent  aqueous  solution  of  potassium  permanganate, 
and  rapidly  decolorized  a  solution  of  bromine  in  chloro- 
form. Unsaturated  hydrocarbons  were  therefore 
present  in  considerable  quantity.  Means  for  the  quali- 
tative detection  of  the  other  classes  of  components  are 
lacking,  however.  Previous  investigators  have  reported 
in  low-temperature  tars  the  following  groups  of  hydro- 
carbons, listed  in  the  order  of  their  chemical  activity: 

1.  Unsaturated    (olefinic  and  cyclic). 

2.  Aromatic   (liquid). 

3.  Naphthene    (saturated  cyclic). 

4.  Paraffine   (liquid  and  solid). 

Determination  of  the  Percentage  of  Paraffine  Plus 
Naphthene  Hydrocarbons  in  the  Neutral  Residues. 
This  analysis  was  accomplished  by  the  treatment  of  15 
to  20  g.  samples  with  75  c.c.  of  98  per  cent  sulphuric 
acid.23  Each  sample  was  agitated  for  half  an  hour  in 
a  glass-stoppered  separatory  funnel,  and  the  weight 
of  the  residual  oils  determined.  The  results  were  cor- 


28H.  G.  Evans,  J.  Soc.  Chem.  Ind.,  vol.  38,  p.  402T   (1919)  ;  F.  B. 
Thole,  ibid.,  vol.  38,  p.  39T  (1919). 

[21] 


rected  for  polymerization"  of  the  unsaturated  hydro- 
carbons by  a  method  based  on  redistillation  of  the 
residual  saturated  oils  and  subtraction  of  a  quantity 
equivalent  to  the  weight  of  the  dissolved  polymers. 
All  determinations  were  made  in  duplicate,  and  the 
means  of  the  closely  agreeing  values  are  plotted  in 
Fig.  2.  The  percentage  of  saturated  hydrocarbons  thus 
obtained  (13.9  per  cent)  is  much  lower  than  any  previ- 


0.660 


120    160     200     240    280 

Average  Boiling  Point,   °C. 

FIG.  9— SP.GR.  OF  SATURATED  HYDROCARBONS 


320 


ously  reported   (Table  II).     This  is  explained  in  the 
section  on  secondary  carbonization  reactions. 

Estimation  of  the  Naphthenes.  The  indices  of  re- 
fraction and  specific  gravities  of  the  redistilled  satu- 
rated hydrocarbons  were  determined.  Fig.  9  shows 
that  the  graphical  relation  between  the  densities  and 
boiling  points  of  these  residual  oils  offers  a  method 
of  approximating  the  proportions  of  paraffines  and 
naphthenes.  The  densities  plotted  for  the  naphthenes 
are  those  reported30  for  hydrocarbons  of  formula  CnH^, 
isolated  from  crude  petroleum.  It  is  apparent  that  the 
position  of  the  Carbocoal  curve  between  the  naphthene 


"B.   T.    Brooks   and    Irwin   Humphrey,  J.    Am.    Chem   Soc.,  vol. 
40,  p.  822   (1918). 

[22] 


and  paraffine  curves  may  be  used  as  a  basis  of  esti- 
mating the  proportions  of  these  groups  of  hydrocarbons. 
No  great  accuracy  can  be  claimed  for  such  an  approxi- 
mation, since  the  densities  of  the  naphthenes  vary 
slightly  according  to  the  nature  of  the  hydrocarbon, 
although  they  are  much  less  erratic  than  the  corre- 
sponding indices  of  refraction.  Representative  values 
were  selected,  however,  and  it  is  believed  that  a  fair 
picture  may  thus  be  obtained  of  the  composition  of  the 
saturated  hydrocarbons  from  low-temperature  tar.  The 
distillate  thus  contained  8.8  per  cent  naphthenes  and 
5.1  per  cent  paraffines. 

An  interesting  sidelight  on  the  nature  of  these  hydro- 
carbons from  low-temperature  tars  is  given  by  a  com- 
parative examination  of  oils  of  the  same  boiling  point 
from  crude  petroleum.  A  kerosene  from  the  mid- 
continental  field  was  washed  thoroughly  a  number  of 
times  with  oleum  until  no  test  was  given  for  unsatura- 
tion.  The  water-white  oil  thus  obtained  after  washing 
with  alkali  and  redistilling  possessed  the  same  fragrant 
odor  as  the  saturated  hydrocarbons.  The  refractive 
indices  and  specific  gravities  (cf.  Fig.  9)  of  the  frac- 
tions of  this  refined  kerosene  reveal  a  striking  simi- 
larity between  these  two  groups  of  oils,  one  obtained 
from  coal  and  the  other  from  petroleum. 

Previous  Investigations.  Previous  investigators  have 
interested  themselves  primarily  in  the  qualitative  study 
of  individual  compounds.  Fischer  and  Gluud31  cooled 
a  dilute  acetone  solution  of  the  oils  to  about  — 70  deg. 
C.  with  liquid  air.  They  estimated  that  the  "benzine" 
fraction  (below  200  deg.),  the  "illuminating  oil"  frac- 
tion (200-300  deg.),  and  the  "lubricating  oil"  fraction 
(above  300  deg.)  each  contained  10  per  cent  of  paraf- 
fines. The  paraffines  boiling  between  200  deg.  and 
300  deg.  were  liquid  (CMHM  to  C19H40)  and  those  in 
the  higher  fractions  were  solid  (C^H..,,  to  CJH^). 

Ultimate  analysis  has  been  employed  by  other  in- 
vestigators for  the  same  purpose.  Jones  and  Wheeler"1 
reported  a  mixture  of  naphthenes  with  a  small  quan- 
tity of  paraffines  in  the  saturated  fraction  35-125  deg. 
C.  of  their  low-temperature  tar,  but  concluded  from 
the  high  percentages  of  carbon  in  the  fractions  boiling 
between  150  and  300  deg.  that  they  "must  consist  of 
naphthenes  only,  or,  what  is  less  likely,  of  mixtures 


30C.  F.  Mabery,  Proc.  Am.  Acad,  Arts  and  Sci.,  vol.  40,  p.  323 
(1914)  ;  Markownikoff  and  Ogloblin,  Berl.  Ber.,  vol.  16,  p.  1873 
(1883)  ;  Konowaloff,  Chem.  Z.,  vol.  14,  p.  113  (1890). 

31F.  Fischer  and  W.  Gluud,  Ges.  Abhandl.,  vol.  2,  p.  295  (1917)  ; 
vol.  3,  p.  39  (1918). 

[23] 


of  paraffines  with  hydrocarbons  richer  in  carbon  than 
the  polymethylenes."  Solid  paraffines  (C^H^  and 
C17HM)  were  crystallized  from  the  fraction  200-260  deg. 
at  30  mm. 

Pictet1  examined  the  six  fractions  of  "vacuum  tar" 
obtained  between  135  and  285  deg.  after  extraction 
with  liquid  sulphur  dioxide,  and  reported  all  to  have 
the  general  formula  of  the  naphthenes.  He  was  able  to 
identify  the  hexahydro-derivatives  of  mesitylene  and 
durene.  The  only  solid  hydrocarbon  found  was  a  naph- 
thene,  melene  (C^H^). 

The  Non-Saturated  Hydrocarbons 

The  unsaturated  and  aromatic  hydrocarbons  con- 
stitute the  second  largest  group  in  the  low-temperature 
tar.  The  distillate  contains  41.5  per  cent  of  these 
compounds,  while  the  tar-acids  amount  to  42.7  per  cent. 
The  low  density  and  congealing  point  of  the  total 
hydrocarbon  mixture  point  to  the  absence  of  ordinary 


0.740. 


/40          ISO          220          260          300 

Average  Bof/ing  Point,   °C. 


340 


FIG.  10— SPECIFIC  GRAVITIES  OF  HYDROCARBONS 
FROM  LOW-TEMPERATURE  TAR 


[24] 


solid  aromatic  hydrocarbons  and  to  the  preponderance 
of  the  unsaturated  components.  The  specific  gravities 
at  20  deg./4  deg.  of  the  non-saturated  fractions  were 
calculated,  as  shown  in  Fig.  10,  from  the  densities  of 
the  total  neutral  oils  and  of  the  saturated  hydrocarbons, 
and  from  the  known  percentages  of  saturated  hydro- 
carbons in  the  neutral  oils.  The  curve  thus  obtained 
is  too  high  for  the  densities  of  the  olefines,  and  indi- 
cates that  the  unsaturated  hydrocarbons  are  cyclic  in 
character. 

Recovery  of  the  Non-Saturated  Hydrocarbons.  The 
only  method  that  appears  to  offer  a  means  of  recover- 
ing the  non-saturated  (unsaturated  and  aromatic) 
hydrocarbons  comparatively  free  from  naphthenes  and 
paraffines  is  extraction  with  liquid  sulphur  dioxide. 
Although  the  fraction  20-173  deg.  is  miscible  with  this 
solvent32  in  all  proportions  at  — 35  deg.  C.,  the  separa- 
tions accomplished  are  sharper  and  more  specific  in 
hydrocarbon  mixtures  of  high  molecular  weight.  While 
non-saturated  hydrocarbons  can  be  recovered  in  this 
manner,  doubt  must  be  cast  on  their  purity,  therefore, 
because  of  the  variation  in  the  effect  of  the  solvent 
throughout  the  wide  range  of  compounds  involved.  In 
particular  it  is  to  be  expected  that  the  lower  fractions 
will  be  found  admixed  with  a  portion  of  the  naphthene 
and  paraffine  hydrocarbons. 

Using  a  ratio  of  three  parts  by  volume  of  oil  to 
five  parts  of  liquid  sulphur  dioxide  and  the  apparatus 
and  method  described  by  Bowrey,33  a  hydrocarbon  mix- 
ture constituting  a  representative  sample  of  the  com- 
bined fractions  was  resolved  into  two  parts;  an  extract 
containing  the  non-saturated  and  a  small  quantity  of 
low-boiling  saturated  hydrocarbons,  and  a  residue  com- 
posed principally  of  saturated  hydrocarbons. 

Examination  of  the  Non-Saturated  Hydrocarbons. 
The  oils  recovered  from  the  sulphur  dioxide  layer  were 
washed  with  alkali  and  subjected  to  three  very 
slow  fractionations  with  a  Vigreux  head.  The  distil- 
lates were  water-white  at  first,  but  after  exposure  to 
the  atmosphere  for  only  a  few  minutes  became  amber 
colored;  the  higher  fractions  showed  a  violet  fluores- 
cence. All  fractions  could  be  distilled  at  atmospheric 
pressure  with  only  slight  decomposition.  The  specific 


™Cf.  R.  J.  Moore,  J.  C.  Morrell  and  G.  Egloff,  MET.  &  CHEM 
ENG.,  vol.  18,  p.  396  (1918). 

MS.  E.  Bowrey,  J.  Inst.  Pet.  Tech.,  vol.  3,  p.  287  (1917)  ;  Ghem 
Trade  J.,  vol.  60,  p.  426  (1917). 

[25] 


CARBOCQAL  (Observed) 

•— "• •-  CARBOCOALfatcutated) 

X x~  VACUUM  TAR  (Pictet) 


140          ISO          220          260          300 

Average  B.  P.  of  Fractions,  °C. 

FIG.    11— SPECIFIC   GRAVITIES   OF  NON-SATURATED 
HYDROCARBONS 


gravities  and  indices  of  refraction  of  the  seventeen 
fractions  collected  were  determined.  Fig.  11  shows 
these  specific  gravities,  both  observed  and  calculated, 
plotted  against  the  average  boiling  points  of  the  re- 
spective fractions. 

It  was  expected  that  admixture  with  saturated  hydro- 
carbons would  cause  the  observed  values  in  the  lowest- 
boiling  fractions  to  fall  below  those  calculated.  The 
calculated  curve  is  too  incomplete  to  establish  this 
point,  but  the  "vacuum  tar"  data  also  plotted  in  the 
figure  show  the  same  effect.  Although  it  is  very  prob- 
able that  low-boiling  saturated  hydrocarbons  were 
present  in  this  extract,  the  large  number  of  fractiona- 
tions  to  which  they  were  subjected  would  be  effective 
in  eliminating  most  of  them  from  the  restricted 
fractions  selected  for  analysis  as  chemical  individuals. 

The  striking  resemblance  between  the  non-saturated 
hydrocarbons  from  low-temperature  tar  and  "vacuum 
tar"  extends  to  the  respective  refractive  indices  and 
average  molecular  weights,  which  were  determined  in 
selected  fractions  by  the  usual  Beckmann  cryoscopic 
method.  The  close  agreement  between  the  physical 
constants  of  these  hydrocarbons  shows  that  the  non- 
saturated  compounds  from  the  former  apparently  be- 

[26] 


long  in  general  to  the  same  class  of  cyclic,  unsaturated 
hydrocarbons  described  by  Pictet.1  In  this  series  he 
was  able  to  identify  dihydro-m-xylene,  dihydromesity- 
lene,  dihydroprehnitene  and  hexahydrofluorene.  The 
higher-boiling  members  of  the  series  have  from  three 
to  five  double  bonds  (basis  of  molecular  refraction), 
and  are  apparently  polycyclic.  No  aromatic  hydrocar- 
bons were  detected. 

Aromatic  Hydrocarbons  in  the  Non-Saturated  Oils. 
The  very  low  congealing  temperatures  of  all  of  the 
fractions  demonstrate  the  absence  of  solid  aromatic 
hydrocarbons  of  the  type  occurring  in  ordinary  high- 
temperature  coal  tar.  Evidence  also  is  not  lacking  to 
indicate  that  no  considerable  percentage  of  liquid 
aromatic  hydrocarbons  can  be  present.  The  densities 
and  indices  of  refraction  of  the  non-saturated  hydro- 
carbons are  notably  lower  than  the  corresponding 
constants  of  aromatic  hydrocarbons  of  the  same  boiling 
point.  Moreover,  close  conformity  has  been  demon- 
trated  between  the  physical  properties  of  the  non- 
saturated  hydrocarbons  of  Carbocoal  tar  and  a  series 
of  unsaturated,  cyclic  compounds  among  which  no 
aromatics  were  detected. 

Summary 

(1)  A    commercial    low-temperature    tar    has    been 
compared   with  coke-oven    and   gas-works    tars.     The 
density  and  "free  carbon"  content  of  the  crude,  dry 
tars  and  the  density,  "free  carbon"  and  air  melting 
point   of  the  pitches   remaining   after  fractional   dis- 
tillation under  similar  conditions  have  been  determined. 
The    values    for    the    low-temperature   tar    have    been 
shown  to  be  notably  lower  than  the  corresponding  fig- 
ures obtained  for  the  coke-oven  and  gas-works  tars. 
The  fractionation  curves  of  the  three  tars,  however, 
are  very  similar. 

(2)  The  phenols  have  been  extracted  from  each  frac- 
tion and  estimated  gravimetrically.     The  percentages 
of  phenol  and  the  cresols  have  been  found  by  methods 
involving  nitration   and  the  determination  of  specific 
gravities    and    freezing    points.      Derivatives    of    the 
naphthols  are  present,  but  a-  and  /3-naphthol  are  absent. 

(3)  The  percentage  of  phenols  in  low-temperature 
tars  increases  with  the  yield  of  tar  from  the  coal  car- 
bonized,   the    quantity    of   non-phenolic   oil   remaining 
relatively  constant.     In  general,  coals  of  the  highest 
volatile  content  yield  the  most  phenols. 

[27] 


(4)  The  nitrogen  bases  are  80  per  cent  tertiary,  20 
per  cent  secondary,  and  contain  no  primary  compounds. 
Both  types  show  unsaturation.     No  preponderant  com- 
pound is  present.     Pyridine  has  been  detected  in  the 
first  fraction,  but  the  higher  boiling  bases  differ  from 
those   in   ordinary  coal   tar  in   that  they   have  lower 
densities  and  higher  molecular  weights.     These  differ- 
ences are  ascribed  to  the  presence  in  the  low-tempera- 
ture   bases    of    partly    hydrogenated    nuclei    and    of 
side-chains    longer    than    those    characterizing    high- 
temperature  bases. 

(5)  Contrary  to  the  "vacuum  tar"  of  Pictet,  low- 
temperature  tar  contains  no  alcohols. 

(6)  Hydrogen  sulphide  is  present  in  the  lowest  frac- 
tions, but  carbon  disulphide  is  absent.     Thiophene  is 
indeterminate  in  the  presence  of  unsaturated  hydrocar- 
bons, which  can  not  be  removed. 

(-7)  The  neutral  oils  derived  from  low-temperature 
tars  are  characterized  by  low  density  and  viscosity, 
and  by  the  absence  of  more  than  traces  of  compounds 
solid  above  — 30  deg.  C.  No  single  hydrocarbon 
preponderates  in  quantity.  Unsaturated  hydrocarbons 
are  present,  and  on  standing  cause  darkening  and 
gradual  increase  in  density  of  the  oils.  By  the  use  of 
98  per  cent  sulphuric  acid,  the  saturated  and  non- 
saturated  hydrocarbons  have  been  separated.  Correc- 
tion is  made  for  polymerization.  The  proportion  of 
naphthenes  has  been  determined  from  the  specific  gravi- 
ties of  the  residual  saturated  hydrocarbons. 

(8)  The  non-saturated  hydrocarbons  recovered  by 
liquid  sulphur  dioxide  have  been  shown  by  determina- 
tions of  density,  index  of  refraction  and  molecular 
weight  to  belong  to  the  same  series  of  cyclic  unsatu- 
rated hydrocarbons  as  those  occurring  in  "vacuum 
tar."  Solid  aromatic  hydrocarbons  are  absent;  liquid 
aromatic  hydrocarbons  can  be  present  only  in  traces. 


[28] 


Examination  of 

Low-Temperature  Coal  Tar 

II.    The  Mechanism  of  Coal  Carbonization 

Critical  Review  of  Current  Theories  of  Secondary  Car- 
bonization Reactions — Interpretation  of  the  Mechanism 
of  Coal  Carbonization  in  the  Light  of  the  Composition 
of  a  Commercial  Low-Temperature  Tar — A  Theory  of 
the  Constitution  of  Coal 

IN  A  previous  contribution  a  scheme  was  outlined 
for  the  ready  examination  of  low-temperature  coal 
tar,  and  by  its  means  the  characteristics  of  a  com- 
mercial product  were  determined.  Comparison  with 
the  composition  of  low-temperature  tars  produced  on  a 
small  scale  shows  that  the  commercial  tar  represents 
a  slightly  more  advanced  stage  of  carbonization.  It 
is  thus  possible  in  the  present  paper  to  present  a  new 
viewpoint  on  the  much-disputed  mechanism  of  the 
carbonization  of  coal,  and  to  interpret  distillation 
reactions  in  the  light  of  the  composition  of  this  inter- 
mediate product.  Finally,  there  is  proposed  briefly  a 
theory  of  the  constitution  of  coal  which  explains  the 
composition  of  low-temperature  tars. 

Berthelot's  classic  theory3*  of  the  origin  of  ordinary 
coal  tar  held  the  field  for  many  years  after  it  was  first 
advanced  in  1866.  Acetylene  is  formed,  Berthelot 
stated,  by  the  decomposition  of  such  simple  gases  as 
methane  in  the  volatile  products  from  coal,  and  poly- 
merizes immediately  at  high  temperatures  to  form 
benzene,  styrolene,  naphthalene  and  other  aromatic 
hydrocarbons.  This  theory  has  been  rendered  unten- 
able, however,  by  experiments  which  have  proved  (1) 
that  acetylene  is  not  formed  in  any  considerable 
quantity  in  the  gaseous  products  of  coal  distilled  at 
various  temperatures,  and  that  its  reaction  velocity  of 
synthesis  is  not  fast  enough  to  account  for  this 
absence;'5  (3)  that  methane,  ethane  and  propane  do 
not  yield  acetylene  as  their  principal  decomposition 


3tBerthelot,  Ann.  chim.  phys.  [3],  vol.  67,  p.  53  (1863)  ;  [4], 
vol.  9,  pp.  413,  455  (1866)  ;  [4],  vol.  12,  pp.  5,  122  (1867)  ;  [4]. 
vol.  16,  pp.  143,  148,  153,  162  (1869). 

35M.  J.  Burgess  and  R.  V.  Wheeler,  J.  Chem.  Soc.,  vol.  97,  p. 
1917  (1910)  ;  vol.  99,  p.  649  (1911)  ;  vol.  105,  p.  131  (1914)  ; 
A.  H.  Clark  and  R.  V.  Wheeler,  ibid.,  vol.  103,  p.  1704  (1913). 

[29] 


products;86  and  (3)  that  primary  distillation  products 
of  relatively  high  molecular  weight — i.e.,  low-tempera- 
ture tars — are  obtained  in  the  carbonization  of  coal. 
All  modern  theories  of  carbonization,  therefore, 
acknowledge  the  important  role  played  by  low-tempera- 
ture tars  in  the  distillation  of  coal.  While  it  is  thus 
generally  recognized  that  ordinary  high-temperature 
(900-1,200  deg.  C.)  coal  tar  is  formed  by  the  decom- 
position of  tar  produced  at  lower  temperatures  (500- 
600  deg.  C.),  the  mechanism  of  this  decomposition" 
is  still  a  matter  of  lively  dispute.  No  comprehensive 
scheme  has  been  advanced  to  account  for  more  than 
relatively  a  small  number  of  carbonization  phenomena. 
Investigators  in  general  have  confined  themselves  to 
limited  aspects  of  the  problem,  and  have  sought  to  lay 
emphasis  upon  certain  predominating  reactions.  Thus, 
the  origin  of  the  aromatic  hydrocarbons  of  ordinary 
tar  has  been  variously  attributed  to: 

(1)  Decomposition  of  the  hydrocarbons  of  low-tem- 
perature tar  to  simple  compounds,  which  subsequently 
undergo  pyrogenetic  syntheses    (Jones,  Bone,  et  al.). 

(2)  Hydrogenation  and  dealkylation  of  the  phenols 
of  low-temperature  tar    (Schulze,   Fischer  and  Schra- 
der). 

(3)  Dehydrogenation   and   dealkylation   of   the    un- 
saturated      hydrocarbons      of      low-temperature      tar 
(Pictet). 

The  present  investigation  leads  to  evidence  which 
supports  in  general  the  last  two  of  these  conceptions, 
and  hence  assigns  to  pyrogenetic  syntheses  a  part  of 
secondary  importance.  A  brief  review  of  the  experi- 
mental data  underlying  these  several  carbonization 
theories  is  a  necessary  preliminary  to  the  development 
of  a  more  comprehensive  and  extended  analysis  of  the 
phenomena  of  coal  distillation. 

THEORIES  OF  DECOMPOSITION  OF  LOW-TEMPERATURE 
HYDROCARBONS 

Pyrogenetic  Syntheses.  Probably  the  most  gen- 
erally accepted  theories  of  tar  formation  at  present  are 
those  which  postulate  the  decomposition  of  the  primary 

S6T.  E.  Thorpe  and  J.  Young,  Proc.  Roy.  Soc.,  vol.  21,  p.  184 
(1873)  ;  H.  E.  Armstrong  and  A.  K.  Miller,  J.  Chcm.  Soc.,  vol. 
49,  p.  74  (1886)  ;  F.  Haber,  Ber.,  vol.  29,  p.  2691  (1896)  ;  W.  A. 
Bone  and  H.  F.  Coward,  J.  Chem.  Soc.,  vol.  93,  p.  1197  (1908). 

slCf.  the  analogous  process  of  decomposition  of  the  bitumen  of 
shale  to  yield  oil  on  distillation,  R.  H.  McKee  and  E.  E.  Lyder, 
J.  Ind.  Eng.  Chem.,  vol.  13,  pp.  613,  678  (1921)  ;  Arthur  J.  Franks. 
CHEM.  &  MET.  ENG.,  vol.  25,  pp.  731,  778  (1921)  ;  C.  W.  Botkin, 
ibid.,  vol.  24,  p.  876  (1921)  ;  L.  C.  Karrick,  Chem.  Age  (N.  Y.). 
voL  30,  p.  112  (1922). 

[30] 


tar  to  simple  unsaturated  hydrocarbons  that  condense 
subsequently  to  form  aromatic  hydrocarbons.  Jones38 
states  that  "the  mechanism  of  the  breaking  down  of 
low-temperature  tar  consists  essentially  in  the  decom- 
position of  the  naphthenes,  paraffines  and  unsaturated 
hydrocarbons  present  in  the  low-temperature  tar  to 
form  defines  of  varying  carbon  content,  which  con- 
dense at  higher  temperatures  to  aromatic  substances." 

In  arriving  at  this  conclusion  he  passed  low-tempera- 
ture tar39  slowly  through  a  tube  filled  with  porous 
porcelain,  and  examined  the  gaseous  products  obtained 
at  various  temperatures  between  550  and  800  deg.  C. 
He  stated  that  benzenoid  hydrocarbons  could  be  formed 
only  to  a  limited  extent40  by  the  elimination  of  hydro- 
gen from  the  corresponding  naphthenes,  since  members 
of  the  cyclohexane  series  break  down  principally  by 
a  scission  of  the  ring  with  the  formation  of  olefines, 
including  butadiene,  and  of  paraffines  and  hydrogen. 
The  gaseous  olefines  were  found  to  be  at  a  maximum 
at  550  deg.,  and  almost  disappeared  at  750  deg.  This 
disappearance  synchronized  with  the  appearance  of 
naphthalene,  and  immediately  preceded  a  rapid  increase 
in  the  evolution  of  hydrogen,  which  "must  probably 
be  attributed  to  the  union  of  the  aromatic  molecules 
and  to  intramolecular  ring  closing." 

Jones  quotes  the  work  of  Staudinger41  as  an  example 
of  the  polymerization  of  diolefines  to  aromatic  sub- 
stances. Moreover,  Staudinger  identified  butadiene  in 
the  gas  distilled  from  coal.  Instances  of  its  formation 
also  from  naphthenes,  olefines,  saturated  hydrocarbons 
and  petroleum  vapors  from  cracking  stills  are  also 
cited.  "It  is  highly  probable,"  Jones  concludes,  "that 
a  necessary  transition  stage  is  the  formation  and  con- 
densation of  olefines  containing  the  conjugated  double 
linkage  — CH :  CH.  CH :  CH —  .  Polynuclear  aromatic 
substances  are  formed  at  750  deg.  and  upward  subse- 
quent to  the  decomposition  of  the  olefines."  Jones,  con- 
trary to  Pictet,  regards  phenols  as  primary  products  of 
coal  distillation,  "those  of  high-temperature  tar  being 
formed  with  certain  changes  from  those  of  low-tem- 
perature tar.  Much  of  the  high-temperature  pitch  is 
formed  by  partial  carbonization  of  the  low-temperature 
pitch." 

38D.  T.  Jones,  J.  Soc.  Chem.  Ind.,  vol.  36,  p.  3   (1917). 

39D.  T.  Jones  and  R.  V.  Wheeler,  /.  Chem.  Soc.,  vol.  105,  p 
140  (1914). 

40D.  T.  Jones,  J.  Chem.  Soc.,  vol.  107,  p.  1582    (1915). 

41H.  Staudinger,  R.  Endle  and  J.  Herold,  Ber..  vol.  46,  p.  2466 
(1913). 

[31] 


Bone,"  from  a  study43  of  the  behavior  of  the  simple 
hydrocarbons  at  temperatures  between  500  and  1,200 
deg.  C.,  has  elaborated  this  theory  of  olefine  condensa- 
tions. He  introduces  the  new  conception  that  the 
thermal  decomposition  of  the  hydrocarbons  of  low- 
temperature  tar  "involves  the  primary  formation  by 
dehydrogenation  of  the  unsaturated  residues  CH2  (two 
free  bonds)  and  CH  (three  free  bonds),  which  during 
a  fugitive  but  really  independent  existence  are  free  to 
interact  with  the  surrounding  gaseous  medium  after 
their  kind."  Bone  points  out  that  the  most  favorable 
temperature  range  for  aromatic  formations  from  such 
residues  (500-800  deg.)  coincides  with  that  giving  the 
best  yields  of  benzene  and  its  homologs  and  is  well 
below  that  which  is  most  favorable  to  the  hydrogena- 
tion  of  the  residues  to  methane.  Between  500  and  800 
deg.  "ethylene  is,  by  reason  of  its  rapid  production  of 
CH  residues  (three  free  bonds)  during  its  primary 
decomposition,  eminently  capable  of  generating  ar- 
omatic nuclei,  although  to  a  less  marked  degree  than 
in  the  case  of  acetylene.  On  the  other  hand,  ethane, 
which  primarily  produces  CH2  residues  (two  free 
bonds)  only,  does  not  form  aromatic  nuclei  so  readily 
as  does  ethylene." 

Another  variation  of  the  olefine  theory  is  advocated 
by  Whitaker  and  Crowell,44  who  distilled  coal  in  5-lb. 
charges  at  different  temperatures,  and  examined  the 
liquid  and  gaseous  products  of  carbonization.  They 
suggest  that  the  course  of  reactions  in  coal  carboniza- 
tion is  as  follows:  "Solid  coal— »high  molecular  weight' 
paraffines— >low  molecular  weight  paraffines;  defines— > 
acetylenes,  naphthenes  and  polycyclic  compounds-* 
benzene  and  its  homologs-^-higher  homologs  of  benzene 
— >xylene— >toluene^benzene,  etc."  Attention  was 
directed  to  the  apparent  marked  similarity  between 
this  series  of  decompositions  and  those  postulated45 
for  oil  cracking.  A  somewhat  similar  sequence  of  coal 
reactions  has  recently  been  advanced  by  Wigersma.46 

^W.  A.  Bone,  "Coal  and  Its  Scientific  Uses"  (London,  1918), 
p.  143  et  seq. 

"W.  A.  Bone  and  H.  F.  Coward,  J.  Chem.  Soc.,  vol.  93,  p.  1197 
(1908). 

"M.  C.  Whitaker  and  W.  H.  Crowell,  J.  Ind.  Eng.  Chem.,  vol. 
9,  p.  261  (1917). 

48C/.  publications  by  Rittman,  Zanetti,  Egloff,  Leslie  ct  al.  in  J. 
Ind.  Eng.  Chem.  A  very  extensive  review  of  the  literature  to 
1916  on  the  pyrogenesis  of  hydrocarbons  is  given  by  E.  L.  Lomax. 
A.  F.  Dunstan  and  F.  B.  Thole,  /.  Inst.  Pet.  Tech.,  vol.  3,  p.  36 
(1916)  ;  c/.  W.  Gluud,  Ges.  Abhandl,  vol.  2,  p.  261  (1917). 

««B.  Wigersma,  Chem.  W«  kbUnl.  vol.  16,  p.  1356   (1919). 

[32] 


THEORY  OF  THE  DECOMPOSITION  OF  LOW-TEMPERATURE 
PHENOLS 

The  investigators  whose  work  has  been  reviewed  thus 
far  have  paid  scant  attention  to  the  role  of  the  phenols 
in  secondary  decompositions.  In  1885  Schulze,47  on  the 
occasion  of  his  pioneer  investigation  of  the  naphthols 
of  ordinary  coal  tar,  stated  that  coal  is  essentially 
altered  cellulose  and  that  its  primary  decomposition 
products  are  the  phenols.  The  phenols  on  further 
heating  (1)  eliminate  water  to  synthesize  high-boiling 
hydrocarbons;  (2)  are  partly  reduced  to  low-boiling 
hydrocarbons;  (3)  are  decomposed  to  form  gases;  or 
(4)  escape  unchanged.  These  reactions  were  conceived 
as  progressing  simultaneously  and  in  equilibrium  with 
each  other.  So  strong  was  the  influence  of  Berthelot's 
theory  at  that  time,  however,  that  Schulze  made  his 
propositions  simply  to  supplement  rather  than  to  dis- 
place the  acetylene  hypothesis. 

Very  recently  Fischer  and  Schrader48  have  also 
assigned  to  the  phenols  a  part  of  major  importance. 
They  point  out  that  low-temperature  tars  consist  essen- 
tially of  hydrocarbons  similar  to  those  of  petroleum 
and  of  phenols,  but  contain  no  aromatic  hydrocarbons. 
The  benzene  homologs  in  high-temperature  coal  tar 
must  be  formed  from  these  phenols,  they  conclude, 
since  the  hydrocarbons  can  be  converted  into  aromatics 
only  to  a  slight  extent.  In  support  of  this  view  experi- 
ments are  cited  on  the  decomposition  of  cresols  and 
xylenols  when  passed  through  tinned  tubes  heated  to 
750  deg.  in  an  atmosphere  of  hydrogen.  The  phenols 
were  reduced  to  benzene  homologs,  and  these  in  turn 
yielded  some  benzene.  Thus,  according  to  these 
authors'  theories  of  the  decomposition  of  low- 
temperature  tar  in  coal  carbonization,  the  aliphatic 
hydrocarbons  are  broken  down  into  gases,  while  the 
hydroaromatic  hydrocarbons  are  in  part  dehydrogen- 
ated  and  in  part  changed  into  gaseous  hydrocarbons. 
A  small  portion  of  the  phenols  of  the  low-temperature 
tar  remain  unchanged,  but  most  of  them  are  reduced 
to  such  stable  hydrocarbons  as  benzene,  or  take  part 
in  syntheses  of  napthalene,  anthracene  and  other 
aromatics. 


47K.  E.  Schulze,  Lieb.  Ann.,  vol.  227,  p.  143    (1885). 
48F.   Fischer  and  H.   Schrader,  Brennstoff   Chem.,  vol.   1,  pp.   4, 
22    (1920)  ;  vol.   2,  p.   37    (1921). 


[33] 


THEORY  OF  DEHYDROGENATION  AND  DEALKYLATION 

Pictet*  passed  "vacuum  tar"  through  a  red-hot  tube 
filled  with  pieces  of  coke,  and  obtained  the  typical 
products  of  high-temperature  distillation.  He  concluded 
that  the  hydroaromatic  hydrocarbons  of  vacuum  tar 
underwent  dehydrogenation  and  detachment  of  side- 
chains  to  yield  aromatic  hydrocarbons  and  the  large 
quantity  of  hydrogen  and  methane  homologs  charac- 
teristic of  high-temperature  gases.  This  theory  is  sup- 
ported by  the  known  tendency  of  aromatic  hydrides  to 
lose  a  part  of  their  hydrogen,  by  the  occasional 
presence  in  ordinary  tar  of  small  quantities  of  hydro- 
aromatic  hydrocarbons,  and  by  the  fact  that  the  crack- 
ing of  naphthenic  petrols  yields  a  certain  quantity  of 
aromatic  hydrocarbons.  Since  cyclohexane  on  cracking 
yields  little  benzene,  and  tends  rather  to  break  the  ring 
with  the  formation  of  unsaturated  aliphatic  com- 
pounds, the  cyclanes  of  coal,  Pictet  states,  cannot  have 
contributed  largely  to  the  formation  of  benzene  and  its 
homologs. 

It  is  the  unsaturated  cyclic  hydrocarbons,  he  argues, 
which  are  present  in  much  larger  quantity,  and  which 
are  hence  the  principal  source  of  the  benzene  hydrocar- 
bons of  ordinary  tar.  Certain  of  the  polycyclic 
hydrocarbons,  such  as  fluorene,  also  owe  their  existence 
to  simple  dehydrogenation,  since  hexahydrofluorene 
was  found  in  "vacuum  tar."  The  origin  of  naphthalene 
and  anthracene  was  regarded  as  still  obscure,  however, 
since  no  hydrogenated  derivative  of  either  of  these  was 
found  in  "vacuum  tar"  or  in  the  benzene  extract  of  coal. 

LIMITATIONS  OF  EAPERIMENTAL  METHODS  IN 
CARBONIZATION  STUDIES 

It  is  desired  to  call  attention  at  this  point  to  certain 
conditions  which  complicate  the  study  of  carbonization 
reactions.  Aside  from  such  factors  of  prime  impor- 
tance as  temperature  and  pressure,  the  course  which 
these  reactions  take  depends  upon  the  relative  concen- 
trations of  the  reacting  substances,  and  hence  the 
nature  of  the  coal  carbonized,  and  upon  the  path  of 
travel  of  the  gases  in  the  oven.80 

Coals  of  oxygen  content  lower  than  normally  used  in 
commercial  carbonization  practice  have  been  employed 
by  some  observers,  who  consequently  lost  sight  of  the 

"A.  Pictet,  Ann.  chim.    [9],  vol.  10,   p.  322    (1918). 
MC/.  G.  E.  Foxwell,  J.  Soc.  Chem.  Ind.,  vol.  40,  pp.  193T.  220T 
(1921). 

[34] 


effect  which  the  higher  phenol  concentration  would 
have  in  the  formation  of  the  typical  aromatic  hydro- 
carbons of  ordinary  tar.  Carbonization  reactions 
occur  simultaneously,  and  represent  complex  equilibria, 
the  individual  factors  of  which  are  difficult  to  divorce 
from  one  another  for  separate  investigation.  When 
experiments  on  the  decomposition  of  isolated  hydrocar- 
bons are  quoted,  translation  of  such  results  to  com- 
mercial conditions  is  obviously  open  to  question.  The 
strongly  reducing  atmosphere  of  coke  ovens  and  gas 
retorts  is  a  factor  that  too  often  has  been  ignored 
in  pyrogenetic  assumptions.  Thus  it  has  been  shown81 
that  at  550  deg.  benzene  in  an  atmosphere  of  nitrogen 
readily  loses  hydrogen  with  the  formation  of  diphenyl, 
but  in  the  presence  of  an  excess  of  hydrogen  yields 
only  traces  of  diphenyl.  Toluene  carried  through  coke 
in  a  stream  of  nitrogen  at  750  deg.  yields  naphthalene, 
anthracene  and  a  viscous  black  liquid.  On  the  other 
hand,  the  substitution  of  hydrogen  for  nitrogen  greatly 
accelerates  the  decomposition  of  the  toluene,  and  pro- 
duces large  amounts  of  benzene  and  only  a  small  quan- 
tity of  solid  condensate. 

Attention  is  directed  also  to  the  difference  in  the 
results  obtained  in  small-scale  experiments  and  in  com- 
mercial operation.  For  example,  when  2-g.  samples  of 
coal  are  distilled52  under  a  high  vacuum  in  a  platinum 
tube,  there  is  obtained  a  maximum  yield  of  tar  at  750 
deg.,  which  is  nearly  four  times  that  recovered  at  450 
deg.  It  is  a  well-known  fact,  on  the  other  hand,  that 
the  yield  of  low-temperature  tars  on  a  commercial 
scale  is  about  double  that  of  ordinary  high-temperature 
tars. 

Again,  Jones38  decomposed  0.1-g.  samples  of  low-tem- 
perature tar  in  a  glass  tube  filled  with  porous  porcelain, 
and  based  much  of  his  theory  of  carbonization  reactions 
on  the  analysis  of  the  gases  obtained  at  different  tem- 
peratures. If  comparison  is  made  of  these  results  with 
the  gas  analyses  of  Thau,53  who  distilled  coal  in  a  com- 
mercial coke  oven  over  the  same  range  of  temperatures, 
striking  differences  are  at  once  manifest.  The  sudden 
increases  in  the  evolution  of  hydrogen  and  of  methane 
synchronizing  with  the  disappearance  of  defines  and 
the  appearance  of  aromatics  noted  by  Jones  is  not 

51 J.  W.  Cobb  and  S.  F.  Dufton,  Chem.  Trade  J.,  vol.  63,  p.  197 
(1918)  ;  Gas  World,  vol.  69,  p.  127  (1918)  ;  Gas.  J.,  vol.  143,  p. 
482  (1918). 

5-M.  J.  Burgess  and  R.  V.  Wheeler,  J.  Chem.  Soc.,  vol.  97,  p. 
1917  (1910)  ;  vol.  99,  p.  649  (1911)  ;  vol.  105,  p.  131  (1914). 

^O.  Thau,  Brennstoff  Chem.,  vol.  1,  pp.  52,  66   (1920). 

[35] 


apparent  in  the  coke-oven  analyses.  As  an  example  of 
the  influence  of  retorting  conditions,  Thau  found  as  a 
result  of  superheating  the  upper  part  of  his  coke  oven 
that  the  hot  surfaces  of  carbon  were  much  more  effec- 
tive in  causing  decomposition  than  were  the  retort 
walls. 

Interpretation  of  Carbonization  Reactions  in  Terms 
of  the  Present  Investigation 

The  low-temperature  tar  studied  in  the  present  in- 
vestigation represents  a  stage  of  carbonization  slightly 
more  advanced  than  that  which  gave  rise  to  the  vacuum 
tars  investigated  by  Pictet  and  by  Jones  and  Wheeler. 
It  was  produced  by  the  Carbocoal  process"  in  a  com- 
mercial retort  that  afforded  an  opportunity  for  sec- 
ondary reactions  to  take  place  to  a  limited  extent. 
Thus  the  distillation  vapors  came  into  contact  with 
the  retort  shell  heated  to  about  600  deg.,  and  yielded 
a  product  that  is  distinguished  from  these  true  low- 
temperature  tars  by  (1)  a  slight  decrease  in  the 
quantity  of  total  phenols  and  a  marked  increase  in  the 
proportion  of  low-boiling  phenols;  (2)  the  appearance 
of  a  large  percentage  of  tertiary  nitrogen  bases;  and 
(3)  a  notable  increase  in  the  ratio  of  unsaturated  to 
saturated  hydrocarbons. 

THE  DECOMPOSITION  OF  LOW-TEMPERATURE  PHENOLS 
DURING  CARBONIZATION 

A  true  low-temperature  tar  contains  about  ten  times 
the  quantity  of  phenols  that  appears  ultimately  in  the 
high-temperature  tar.  Thus,  a  30  per  cent  volatile  coal 
produces  a  primary  tar  containing  about  one-quarter 
its  weight  of  phenols,  and  yields  in  the  coke  oven  about 
half  this  weight  of  tar  containing  less  than  5  per  cent 
of  phenols.  The  low-temperature  phenols  consist  prin- 
cipally of  the  higher  homologs.  High-temperature 
phenols,  on  the  other  hand,  contain  about  two  parts  of 
cresols  to  one  of  phenol,  together  with  a  relatively 
insignificant  proportion  of  higher-boiling  tar-acids. 
It  is  thus  apparent  that  while  the  phenolic  components 
of  the  primary  tar  contribute  very  largely  to  the  forma- 
tion of  secondary  products,  only  a  small  part  of  them 
survive  as  phenols,  and  then  chiefly  as  the  first  mem- 
bers of  the  series. 

The  low-temperature  phenols,  as  would  be  expected. 


61See  p.  6  for  a  description  of  the  conditions  of  carbonization  in 
the  Carbocoal  process. 

[  36  ] 


occupy  a  position  intermediate  between  the  extreme 
low-temperature  type  and  the  phenols  of  ordinary  tar. 
The  phenols  obtained55  by  steam  distillation  of  a  Loh- 
berg  gas  coal  at  a  temperature  slightly  lower  than  that 
in  the  case  of  the  low-temperature  phenols  exceeded  the 
latter  in  total  quantity.  The  fraction  of  Lohberg 
phenols  distilling  below  230  deg.  constituted  only.  7  per 
cent  of  the  total  weight,  while  the  corresponding  frac- 
tion of  low-temperature  phenols  amounted  to  57  per 
cent.  It  thus  becomes  evident  that  the  first  stage  in 
the  decomposition  of  the  primary  phenols  is  one  which 
involves  principally  the  elimination  of  the  multiple 
short  sidechains  of  the  higher  homologs.  This  reaction 
is  probably  accomplished  by  the  replacement  of  the 
methyl  or  ethyl  groups  by  hydrogen  from  the  retort 
gases,  with  the  formation  of  methane  or  ethane. 

Again,  when  low-temperature  phenols  are  passed 
through  a  tinned  tube  at  750  deg.  in  an  atmosphere 
of  hydrogen,  there  are  obtained48  low-boiling  phenols 
and  hydrocarbons,  together  with  water.  Higher  tem- 
peratures, therefore,  must  eliminate  the  hydroxyl 
groups  also,  and  the  predominating  reaction  then  be- 
comes hydrogenation  with  the  production  of  water. 
Moreover,  cresol  when  passed  over  coke  at  750  deg. 
in  a  stream  of  hydrogen  yields  benzene  and  toluene.51 

The  present  investigation  has  shown  that  the  low- 
temperature  phenols  contain  about  10  per  cent  of 
naphthol  derivatives.  The  hydrogenation  of  these  with 
elimination  of  water  is  obviously  one  source  of  the 
naphthalene  of  high-temperature  tar. 

If  the  decomposition  of  the  primary,  monocyclic 
phenols  were  confined  to  such  hydrogenation  reactions, 
however,  there  would  result  in  high-temperature  tar  a 
much  higher  yield  of  light  oils  and  solvent  naphtha 
than  is  actually  the  case.  Experiment  has  shown51 
that  monocyclic  aromatic  hydrocarbons  also  undergo 
the  same  reaction — e.g.,  xylene  is  converted  into 
toluene,  and  toluene  is  converted  into  benzene,  with 
the  simultaneous  formation  of  a  small  amount  of  solid 
condensate.  Thus  it  was  found44  that  the  temperatures 
of  maximum  formation  of  xylene,  toluene  and  benzene 
are  respectively  600,  700  and  800  deg.,  and  that  the 
total  quantity  of  light  oil  formed  decreases  steadily  as 
the  temperature  of  carbonization  is  increased  above 
500  deg.  The  limited  quantity  of  benzene  itself  occur- 
ring in  high-temperature  tar,  however,  makes  it  neces- 


55F.  Fischer,  Brennstoff  Chem.,  vol.  1,  pp.   31,  47    (1920). 

[37] 


sary  to  conclude  that  in  addition  to  the  reactions 
enumerated,  a  portion  of  the  benzene  participates  in 
pyrogenetic  syntheses.  It  is  possible  also  that  some 
of  the  phenols  sustain  scission  of  the  ring  and  are  lost 
from  the  tar  as  gases,  or,  as  Fischer  and  Schrader48  sug- 
gest, condense  to  form  polycyclic  aromatic  hydrocar- 
bons. However  this  may  be,  it  is  nevertheless  evident 
that  the  phenols  are  the  chief  source  of  the  monocyclic 
aromatic  hydrocarbons. 

THE  DECOMPOSITION  OF  THE  NITROGEN  BASES 
DURING  CARBONIZATION 

Since  a  considerable  quantity  of  the  nitrogen  bases 
of  low-temperature  tars  remains  after  distillation  in 
the  pitch,  it  is  difficult  to  determine  whether  the  total 
quantity  is  greater  or  less  than  in  ordinary  tar.  It 
was  found,69  however,  that  the  percentage  of  bases 
(3.42  per  cent)  boiling  under  326  deg.  in  a  mixture  of 
coke-oven  and  gas-works  tar  was  nearly  double  that 
(1.94  per  cent)  of  the  low-temperature  tar.  It  is 
probable,  therefore,  that  low-boiling  bases  are  formed 
from  high-boiling  bases  in  a  manner  analogous  to  the 
decomposition  of  the  high-boiling  phenols. 

Pictet's  suggestion49  that  quinoline  is  formed  from 
dihydroquinoline  by  simple  dehydrogenation  seems  to 
be  the  most  rational  explanation  of  the  primary  decom- 
positions of  the  low-temperature  bases.  Secondary 
bases  of  the  type  of  dihydroquinoline  are  found  in 
"vacuum  tar,"  but  tertiary  bases  are  absent.  The  fact 
that  80  per  cent  of  the  low-temperature  bases  and 
almost  all  of  the  high-temperature  bases  are  tertiary 
compounds  supports  this  theory  of  dehydrogenation. 

Experiments  with  the  low-temperature  bases  demon- 
strated the  presence  of  unsaturated  bases,  the  relative 
molecular  weights57  of  which  average  10  to  15  higher 
(Fig.  12)  than  the  values  of  the  corresponding  high- 
temperature  compounds.  At  higher  temperatures  (700 
to  800  deg.),  therefore,  a  process  of  combined  dehydro- 
genation and  dealkylation  must  take  place  to  yield  the 
ordinary  bases  such  as  quinoline  and  acridine.  More- 
over, the  toluidines  described  by  Pictet  then  yield  the 
aniline  of  ordinary  tar  just  as  it  has  been  shown  that 
cresol  yields  phenol,  and  toluene  yields  benzene. 


MSee  first  paper  in  this  series. 

"Only  relative  values  are  given  by  the  ordinary  Beckmann 
cryoscopic  method,  since  nitrogen  bases  are  slightly  associated  in 
benzene  solution. 

[38] 


The  low-temperature  distillate  was  shown  to  contain 
no  preponderating  base,  but  to  represent  a  series  of 
compounds  of  increasing  molecular  weight.  The  dis- 
tillation curves  of  Fig.  13  emphasize  the  preponderance 
of  quinoline  in  the  high-temperature  bases,  and  indi- 
cate, if  this  theory  is  accepted,  that  the  bulk  of  the 


Average  Mol.  Weight 
l^l^lllslil 

0 

p^^ 

/ 

1 

c& 

rboc 

~M/- 

\/ 

, 

y/^ 

Te/q 

— 

r 

& 

•• 

' 

V* 

o—  — 

4- 

—  • 

"140        160          220        260        300        340 

Average  B.  R  of  Fractions,  CC. 

FIG.  12— AVERAGE  MOLECULAR  WEIGHTS  OF  BASES 


middle-boiling  bases  of  low-temperature  tar  are  struc- 
tural derivatives  of  quinoline,  and  yield  this  compound 
upon  decomposition  at  the  temperature  of  the  coke 
oven.  The  physical  constants  obtained  are  consistent 
wtih  this  observation,  and  dihydroquinoline  has  been 
identified  in  "vacuum  tar." 

THE  DECOMPOSITION  OF  THE  HYDROCARBONS 
DURING  CARBONIZATION 

The  ratio  of  saturated  to  unsaturated  hydrocarbons 
is  highest  in  the  tars  produced  under  high  vacuum  at 
the  lowest  temperatures.  Thus  the  saturated  hydrocar- 
bons obtained  by  Jones  and  Wheeler  were  about  equal 
in  quantity  to  the  non-saturated,  while  in  low-tempera- 
ture tar  they  formed  only  one-third  of  the  distillate 
and  a  much  smaller  fraction  of  the  entire  tar.  The 
obvious  conclusion,  therefore,  is  that  the  initial  decom- 

[39] 


position  of  the  primary  hydrocarbons  is  characterized 
by  a  partial  dehydrogenation  of  the  naphthenes  to 
form  hydroaromatics. 

As  mentioned  above,  Jones40  was  led  to  believe  from 
his  experience  in  the  decomposition  of  cyclohexane, 
methylcyclohexane  and  the  di-  and  tetrahydro-deriva- 
tives  of  naphthalene  that  the  naphthenes  cannot  be 
regarded  as  the  direct  source  of  the  high-temperature 
aromatic  hydrocarbons  through  simple  dehydrogena- 


10    20  30  40    50    60    70    80   90   100 

Percent.of  Total  Wume  Distilled 

FIG.  13— DISTILLATION  OF  BASES 


tion  and  dealkylation.  The  large  yields  obtained  of 
both  the  corresponding  aromatic  hydrocarbon  and 
hydrogen  were  explained  on  the  basis  that  "the  most 
probable  course  of  the  reaction  would  be  for  the  cyclo- 
hexane first  to  lose  two  atoms  of  hydrogen  and  form 
cyclohexene,  which  then  decomposes  in  two  ways,  yield- 
ing benzene  and  butadiene."  The  principal  reaction 
and  the  one  contributing  most  largely  to  aromatic 
formation  was  held  to  be  the  condensation  of  such  con- 
jugated unsaturated  nuclei  as  the  butadiene  represents. 
These  nuclei  are  stated  to  result  in  the  distillation  of 
coal  from  the  decomposition  of  the  paraffines,  the 

[40] 


naphthenes  and-the  unsaturated  hydrocarbons  of  low- 
temperature  tar. 

Results  in  the  present  investigation,  on  the  con- 
trary, lead  to  the  contention  that  while  the  elimi- 
nation of  hydrogen  from  the  naphthenes  of  the  primary 
low-temperature  tar  is  the  initial  step  in  the  decom- 
position reactions,  the  intermediate  di-  and  tetra-hydro- 
derivatives  thus  formed  constitute  a  distinct  and 
comparatively  stable  stage  in  the  carbonization  of  coal. 
Consistent  with  his  theory  Jones  considered  these 
compounds  to  be  partly  olefinic.  Pictet,  on  the  other 
hand,  identified  them  as  unsaturated  naphthenes,  and 
advanced  the  rational  hypothesis  that  the  origin  of  the 
aromatics  lay  in  a  process  of  dehydrogenation  of  the 
aromatic  hydrides  and  of  detachment  of  sidechains 
from  alkyl  derivatives.  The  composition  of  the  low- 
temperature  hydrocarbons  has  been  shown56  to  conform 
with  this  hypothesis  of  Pictet  and  to  be  consistent  with 
all  the  experimental  facts  upon  which  Jones  bases  his 
interpretation. 

THE  ORIGIN  OF  NAPHTHALENE  IN  COAL  TARS 

The  bulk  of  the  unsaturated  hydrocarbons  of  low- 
temperature  tars  are  polycyclic.  While  some  of  the 
naphthalene  of  ordinary  tar  results  from  the  hydro- 


200 
190 


110 


100 


> 

y 

r 

, 

/ 

v 

/ 

j 

^ 

/ 

Car 

•^^( 

7tf/ 

^S 

/ 

Naphthalt 

V7« 

c 

X 

x^ 

>W          /^7  180  220          260          300 

Average  Boiling  Point,  °C. 

FIG.  14 — MOLECULAR  WEIGHTS  OF  NON- 
SATURATED  HYDROCARBONS 


[41] 


genation  of  naphthols,  it  is  believed  that  the  naphthalene 
and  other  polynuclear  aromatic  hydrocarbons  owe  their 
origin  chiefly  to  the  decomposition  of  these  polycyclic 
unsaturated  compounds.  The  presence  of  a  series  of 
homologs  of  hydrogenated  and  alkylated  bicyclic  hydro- 
carbons is  indicated  in  low-temperature  tar  by  the 
physical  and  chemical  properties.  For  example,  the 
average  molecular  weights58  of  the  middle-boiling  frac- 
tions (Fig.  14)  are  notably  higher  than  that  of 
naphthalene.  Fig.  15  shows  that  the  secondary  reac- 
tions giving  rise  to  high-temperature  tar  decompose 
these  bicyclic  hydrocarbons  to  the  single  stable  naph- 


0     10     20    30    40    50    60    70     80    90    100 

Percent  Total  Volume  Distilled 

FIG.    15— DISTILLATION  OF  HYDROCARBONS 


thalene  nucleus,  the  presence  of  which  in  large  quantity 
is  one  of  the  outstanding  features  of  ordinary  tar.  The 
analogy  between  naphthalene  and  quinoline  in  chemical 
structure  makes  more  striking  the  similarity  between 
the  carbonization  reactions  of  the  hydrocarbons  and 
nitrogen  bases. 


D8Determined   in   benzene   solution   by   the  Beckmann   cryoscopic 
method. 

[42] 


A  THEORY  OF  THE  CYCLIC  STRUCTURE  OF  COAL 

Probably  the  most  remarkable  general  characteristic 
of  low-temperature  tars  is  the  fact  that  they  are  almost 
entirely  cyclic  in  nature.  So  far  as  the  various  com- 
pounds have  been  identified,  moreover,  these  cycles  have 
shown  themselves  to  be  six-membered  rings  or  com- 
binations of  six-membered  rings.  This  structural 
characteristic  apparently  persists  in  the  tar  as  the 
temperature  of  distillation  of  the  coal  is  increased. 

It  seems  reasonable,  therefore,  in  view  of  the  great 
stability  of  the  six-membered  ring,  to  ask  if  this  struc- 
ture is  not  characteristic  also  of  the  coal  substance 
itself.  Such  a  conception  is  contrary,  of  course,  to  the 
widely  accepted  cellulose-furane  theory.88  Fischer  and 
Schrader60  have  recently  stated  that  the  plant  cellulose 
is  destroyed  by  bacterial  activity  in  the  early  stages 
of  peat  formation,  and  pointed  out,  moreover,  that 
while  cellulose  yields  on  decomposition  almost  exclu- 
sively phenol  itself,  low-temperature  tars  contain 
chiefly  the  higher  homologs  of  phenol.  In  the  absence 
of  cellulose  the  lignin  must  hence  be  the  source  of  the 
phenols  of  coal  tar. 

The  cyclic  structure  of  lignin  has  been  advocated  by 
these  authors,  who  assign  to  it  an  aromatic  configura- 
tion with  acetyl  and  methoxyl  groups.  During  coal 
formation  the  methoxyl  content  must  eventually  de- 
crease through  saponification,  reduction  or  replacement 
by  hydroxyl  groups.  In  any  case  there  results  a  phenol, 
which  is  considered  identical  with  mimic  acid.  Oxida- 
tion or  polymerization  of  humic  acid  forms  the  alkali- 
insoluble  humin.  Further  splitting  off  of  water, 
carbon  dioxide  and  perhaps  methane  at  ordinary  tem- 
perature, these  authors  argue,  leads  to  lignite  and  coal. 
The  whole  series  is  thus  assumed  to  perpetuate  the 
aromatic  structure  of  lignin.  The  production  of  large 
amounts  of  mellitic  acid  by  the  pressure  oxidation  of 
charcoal  and  ordinary  coals  (but  not  of  cellulose)  is 
contributory  evidence  of  the  presence  of  six-membered 
cycles  in  coal. 

The  present  investigation  leads  to  the  suggestion, 
therefore,  that  the  chemical  properties  of  coal  may  be 


59D.  T.  Jones  and  R.  V.  Wheeler,  J.  Chem.  Soc.,  vol.  105,  pp. 
140,  2562  (1914)  ;  vol.  107,  p.  1318  (1915)  ;  vol.  109,  p.  707 
(1916)  ;  cf.  also  "Monograph  on  the  Constitution  of  Coal,"  by 
M.  C.  Stopes  and  R.  V.  Wheeler  (London,  Dept.  Sci.  and  Ind. 
research,  H.  M.  Stationery  Off.)  for  an  exhaustive  review  of  the 
knowledge  on  this  subject  in  1918. 

«°F.  Fischer  and  H.  Schrader,  Bremistoff  Chem.,  vol.  2,  p.  37 
(1921)  ;  cf.  Klever,  Jonas  and  Keppeler,  ibid.,  vol.  2,  p.  213 
(1921),  and  Fischer  and  Schrader,  ibid.,  vol.  2,  p.  237  (1921). 

[43] 


represented  by  a  structure  containing  many  such 
cycles.  This  structure  may  be  pictured  as  an  aggre- 
gate of  "mosaics"  of  these  rings.  Some  of  the 
"mosaics"  contain  oxygen,  and  display  the  insolubility 
characteristic  of  the  "cellulosic  degradation  prod- 
ucts,"88 or,  according  to  Fischer  and  Schrader,  the 
"lignin  degradation  products."  These  may  be  regarded 
as  polymerized  phenols,  as  suggested  above,  or  as 
multi-molecular  structures  in  which  component  rings 
are  joined  together  by  oxygen-containing  bridges. 
Heating  results  in  the  breaking  of  these  bridges  with 
the  formation  of  high-boiling  phenols,  the  sidechains 
of  which  are  remnants  of  other  broken  linkages. 

Similarly,  other  "mosaics"  may  be  pictured  to  rep- 
resent the  same  typical  rings  held  together  by  bridges 
of  paraffine  hydrocarbons,  the  breaking  or  detachment 
of  which  are  responsible  for  the  presence  of  straight- 
chain  hydrocarbons  in  low-temperature  tars,61  or  even 
in  seams  of  the  coal  deposits  themselves.  These  struc- 
tures are  more  soluble,  and  have  been  called  the 
"resinous  constituents."5*  Nitrogen  and  sulphur  com- 
pounds may  be  regarded  as  appearing  in  strictly 
analogous  configurations. 

SUMMARY  OF  THE  MECHANISM  OF  COAL  CARBONIZATION 

(1)  The    decomposition    of    the    coal    substance    to 
ordinary  high-temperature  tar  when  subjected  to  the 
action  of  heat  is  a  process  of  progressive,  step-by-step 
decomposition,  in  which  pyrogenetic  syntheses  play  only 
a  secondary  part. 

(2)  Six-membered   rings   and   combinations   thereof 
characterize  the  entire   series   of   decomposition   prod- 
ucts from  coal  to  high-temperature  tar.     The  decompo- 
sitions  during  carbonization   are   essentially   reactions 
effecting  the  elimination  of  sidechains. 

(3)  The   average   molecular   weights    of   the   liquid 
intermediate  products  constantly  decrease  as  the  tem- 
perature   of    carbonization    rises.      This    decrease    is 
marked   by    the   evolution    of   hydrogen,    methane    and 
ethane. 

(4)  The   initial   decomposition   of   the   low-tempera- 
ture tar  first  formed  is  brought  about  by   (a)  loss  of 
hydrogen  from  a  portion  of  the  naphthenes  with  a  re- 
sultant   increase    in    the    proportion    of    unsaturated 
hydrocarbons;    (6)  loss  of  sidechains  from  the  phenols 


C1C7.  the  "bound  molecules"  of  Jones  and   Wheeler   (Ref.   59). 

[44] 


by  hydrogenation  with  the  resultant  formation  of 
lower-boiling  phenols;  and  (c)  loss  of  hydrogen  from 
the  nitrogen  bases  to  form  a  large  proportion  of  ter- 
tiary compounds.  (This  stage  is  represented  by  Car- 
bocoal  tar.) 

(5)  Final    decompositions    are    at    a    maximum   be- 
tween   700    and    800    deg.,    and  'are    marked    by    (a) 
dehydrogenation    and    dealkylation    of    the    hydroaro- 
matic  unsaturated  hydrocarbons  and  nitrogen  bases  to 
form    aromatics,    with    the    elimination    of    hydrogen, 
methane   and   other   simple   gases;    (Z>)    hydrogenation 
of  the  phenols  to  aromatic  hydrocarbons  and  of  these 
aromatic  hydrocarbons  to  lower-boiling  aromatics,  with 
the  formation  of  methane,  ethane  and  water;  and   (c) 
secondary   pyrogenetic   syntheses   of   higher   aromatics 
from  simple  compounds. 

(6)  The    phenols    of    low-temperature    tar    are    the 
principal  source  of  the  monocyclic  aromatic  hydrocar- 
bons.    The  unsaturated  naphthenes  of  low-temperature 
tar  are  the  principal  source  of  the  polycyclic  aromatic 
hydrocarbons. 

Department  of  Chemical  Engineering, 

Columbia  University  in  the  City  of  New  York. 


45 


Vita 

Roland  P.  Soule  was  born  in  Rochester,  N.  Y., 
January  17,  1896.  His  primary  and  intermediate 
education  was  obtained  in  the  public  schools  of  that 
city.  In  June,  1917,  he  received  the  degree  of 
Bachelor  of  Science  from  the  University  of  Rochester 
with  honors  in  the  Department  of  Chemistry.  The 
following  summer  was  spent  in  Baltimore,  Md.,  where 
he  was  employed  in  the  Research  Laboratory  of  the 
U.  S.  Industrial  Alcohol  Company.  He  entered  the 
School  of  Mines,  Engineering  and  Chemistry  of 
Columbia  University  in  September,  1917,  and  received 
the  degree  of  Chemical  Engineer  in  1920.  In  the  sum- 
mer of  1918  and  in  the  academic  year  of  1919-20  he 
was  Assistant  in  the  Department  of  Chemical  Engineer- 
ing. He  was  employed  in  the  Research  Laboratory  of 
the  National  Biscuit  Company  in  the  summer  of  1919. 
He  became  a  University  Fellow  in  September,  1920,  and 
received  the  degree  of  Master  of  Arts  in  1921.  During 
the  past  year  he  has  been  an  incumbent  of  the  S.  W. 
Bridgham  fellowship. 


[46] 


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